A mystery problem keeping corys

[Rebdg]

Please help... 40 gal planted aquarium ( 36 x 12 x 18 ) with florite substrate and one piece of very aged drift wood. 3 very small petrified wood rocks. Aquarium is low light meaning no co2 is used but ferts are used in small amounts. Mostly Seachem florish, trace, and potassium. Temp is 79 degrees F. ph is 6.0 Gh is 6 dh and Kh is 1 or less. Ammonia nitrates and nitrites test at 0. Filter is Eheim 2215 with spray bar. Media in filter is mechanical and biological... no chemical filtration is used for the aquarium. This set up has been running for over one year. Water changes are 10% every 3 weeks. This is a community tank with tetra's, ottos, an sae, a pleco and a pair of black angles that have been breeding every 4 weeks and raising young to free swiming before the babies are eaten. All the fish in the tank have florished and grown. There are also nerite snails. No meds are used as inhabinates are healthy.
My problem is this: I have never been able to get corys to live in this aquarium for more than a week. Over the past year I have introduced 25 or more different types of corys at varying times usually 3-5 at a time. They never last more than 7 days. The corys will school and eat for one to two days ... then act very stressed with rapid breathing. They do get to the top for air but eventually start acting very disoriented, move more slowly, swim irregularly, and then die. I have bought corys from several places and the results are the same. All the corys in the stores continue to thrive.I am aware that florite substrate is not the best for corys but I see them thriving in a planted show tank in one of the stores I use. When a youngster I kept corys very well in basic starter tanks. I really love these fish but can't understand why they will not live in my tank when all other fish do so well. I put cardinal tetra's in the tank a year ago and they are still thriving so why not the corys? A baby gold nugget pleco is growing and doing well but again the corys I bought with him have died. I hope the forum can help me understand why no cory has been able to live in this aquarium. Thank you.

[wrasse]

2 things strike me-
1 - a 36 x 12 x 18"high tank holds just under 25 imperial gallons, not 40.
2 - zero nitrates is an odd reading in a fresh water tank.

From the first point - are you adding fertilizers for a 40 gallon tank? If so you are over-dosing.
Second point - Nitrates are the end result of the nitrogen cycle and its reduced mainly by water changes and also by plants absorbing it, also chemicals can be used. You do water changes only every 3 weeks and its not heavily planted. So maybe your test kits are 'faulty'?
And also double check your PH. All your other fish are happy in warm acidic water. Your newly added corys might be struggling to adapt to it... the symptoms you describe could indicate acidosis, although this usually would show up quicker.
A big water change the day before you add new fish will help them adapt.

Hope that helps.

[jimoo]

Are you using RO water? I had problems like what you're describing when I was using too much RO water (without adding buffer) and there were PH spikes. The cory's seemed to take the brunt of it. No enough or frequent enough water changes, either, IMO.

[Rebdg]

Thanks for the replies thus far... the tank is 38 us gallons... the height measurment I used is off.... more like 21 -23 inches I quess. I do not use RO water but do use a tap water filtration system and add buffers back. I use fertalizers at half the strength recommended for a tank my size. But I agree that more frequent water changes will help and perhaps I will stop fertalizing for several weeks to determine if that is the cause. I will also try a new nitrate test kit but I was assuming the plants are using the nitrate. For a low light tank it is very well planted and growing many plants well without algae. Any thoughts on getting more filtration on the tank also? Appreciate the feedback on all this.... Steve

[jimoo]

I have the same size tank for my community tank, and it's fully planted. I only fertilize lightly every 3-4 months. It really doesn't seem to be needed. The tank growth is very dense without it. No C02 injection either.

[nj2tou]

Hmmm... 79 degrees seems a bit high for cories. I keep my main tank at around 74 degrees and the tank with my betta around 75-76. I know the betta would probably prefer 78 or so, but they're ok in slightly cooler temperatures. Also, ph of 6 might stress them out if they're used to closer to 7. it might be too big a drop for them, unless they're gradually acclimated. What type of cories are you getting?

Those are the two things that stand out to me, but I'm pretty new with cories too, so these are just ideas from me.

[MatsP]

pH 6.0 shouldn't be a problem for nearly all corys, as long as you don't keep them in pH 8.0 first and then move them directly to pH 6.0 without letting them settle. My RIO400 tank has on a few occasions dropped to pH 4.0 (not my plan, but with soft water and high bioload, this stuff happens if I'm not on the water changes regularly), and my corys in that tank is perfectly fine.

From what I can see in this thread, the corys are just identified as "corys", so there is no telling whether they are warmer or cooler water species, nor if they are easy or difficult species to keep. Most corys are indeed easy to keep, but some are not. Some corys come from breeders that don't look after them very well either - Corydoras panda appears to be one that isn't the easiest to keep.

The fact that the KH is "1 or less" is a cause for concern about the pH level - this is exactly the problem that causes by RIO400 tank's pH to drop to 4.0 - KH goes to zero. If I add some Salifert KH + pH buffer (there are many other companies selling similar stuff, including supermarkets selling "Sodium bicarbonate"), the pH stays much more stable.

So, in summary, there are several possible causes:
1. pH crash. Fix by adding KH buffer.
2. Temperature. This is more difficult, as the gold nugget for example is not tolerant to temperatures much lower than 80'F. Perhaps try getting some corys that like warm water - Corydoras sterbai, for example.
3. Poor quality stock. Perhaps you can try a different form of cory, or buy from another shop?

--
Mats

[nj2tou]

Well, now that I think of it, I had had the same problem recently. I got 8 pandas in great health from a good breeder, put them in my new tank and one by one 5 of them died. It could have been the shock of the different water parameters and temp. I tested the water and did daily partial water changes as the tank finished cycling. I don't recall there being ammonia and nitrite spikes or any signs of stress or illness in the fish, would just find one dead. I thought I was going to lose them all, but then the deaths just stopped. The remaining 3 plus the 3 from my other tank and the peppereds and metae are now all in that tank and doing great! Haven't lost any in some time now.

So again, perhaps it's a combination of the high temperature and low ph and different water that stresses them too much. My tank originally was closer to 78 or so and it was still cycling, so I'm sure that was a factor. I do want to know what kind of cories he has, as we know some are more sensitive than others.

[Rebdg]

Many many thanks to all who have provided thoughts and insights to my problem keeping corys alive in my aquarium. Bought some additional test equipment to recheck my water and got different results.... ph comes in at 6.6 Ammonia at 0 and here's a problem.... nitrates are over 100. Two partial water changes have done almost nothing to lower it thus far and I quess I can conclude that the tank is just too full of decaying plant matter as well as some overcrowding due to the size of angel fish that have grown very large. I suspect my eheim 2215 is not powerful enough to circulate water and flush waste from the planted tank...perhaps I will get the 2217... I also lowered the temp to 77. I have one sterbai surviving how I don't know. He is still very stressed with rapid breathing. Before I get any more cory's I will work on getting the nitrates much lower. I hope the new test is the more accurate test.I also want to see if there are ph drops and swings. Is there any advise on lowering nitrates safely besides cleaning and water changes and removing the two angel fish? Thanks, Steve

[Rebdg]

Here is the list of cory's tried in my tank:
Panda's
aneus
paleatus
trillineatus
sterbi
matae

Steve

[MatsP]

It is a known fact that nitrate tests can go off over time, leaving them showing low levels of nitrate when the level is much higher. Does your nitrate test go to 100, or is there another level above 100 ppm that it can show? What I'm trying to say is, have you reached the max of the test?

What is the nitrate level of your tap-water? If you are doing water changes and the new water is 50 ppm, then it will take a lot of water changes to lower 100 ppm to something more tolerable.

Also, replacing your filter will do absolutely NOTHING to reduce the nitrate level. The nitrate comes from the breakdown of proteins into ammonia, which is converted by your filter bacteria to nitrite, which another type of bacteria converts to nitrate. In nature, denitryfying bacteria then produces nitrogen gas. This however, happens in anoxic conditions (an environment with no oxygen), which is very bad in a fish-tank. Without either expensive nitrate removal resins or expensive de-nitrifying equipment [something few hobbyists have ever successfully constructed - it's essentially a filtration system which produces anoxic conditions], the only way to get rid of nitrate is to remove the high-nitrate water and replace it with low nitrate water.

Feeding less, and removing deteriorating plants is also a good plan. But moving the plant matter into the filter will have no benefit on the nitrate level at all.

--
Mats

[Rebdg]

Thanks for your post Mats, my tap water filter provides zero nitrate water and the test I have goes up to 180. I may have come down a bit in nitrates from a couple water changes but what I am learning is that I have a very dirty tank in terms of plant waste plus I need to be way more consistant with water changes . I plan to clean dead leaves from the plants and vacuum the substrate as well as then clean out the filter. I was thinking of adding a hang on filter during the clean out with some type of medium to remove nitrate also . This would be a temp solution untill I can get into an acceptable range, What I understand now is that my system is producing way more nitrate as an end product then the plants are using. I will reduce the number of fish and do more frequent water changes. Perhaps this will produce better conditions for cory's to survive. I started off having a planted tank with a few fish to probably have too many and tipping the scale in the wrong direction. My older test was not working I thought all was well.

[MatsP]

Thanks for your post Mats, my tap water filter provides zero nitrate water and the test I have goes up to 180.

Have you double-checked the nitrate in your tap-water?

Sounds like you have a plan, and I see nothing wrong with it.

--
Mats

[Rebdg]

Ok Mats I checked the tap water and of course it measured 0 so I will have to reconsider just what the tap water filter is doing for me...Thanks for your insights By the way I have 1 sterbi left and he continues to hang on.... Fish load reduced....water changes... and started using a nitrate reducer just until aqaurium nitrate are at a safer level.

[shannon]

More airation may help, warmer water has less disolved o2. It was your statement abut your serving fishe's rapid breathing that makes me think it may help. I don't think it is the major factor but it can only help, espicaily at night, when the plants are using it up.

[Rebdg]

After discovering my aquarium nitrate level was out of control-- I have gotten down to the 20's from at least 180. I have reduced the number of fish in my aquarium and cleaned the top of my substrate of plant debris as well as cleaning debris out of the canister filter. I am using a product that further reduces nitrate in a small internal filter I am temporarily using. I have done water changes also.

I know I need to drop nitrates further but I am wondering the ideal range for cory's especially sterbai.

Also how many small fish ( tetra's) would one put in a 40 gal heavily planted aquarium that already has 4 small oto's , a large sae, and 3 rams. Please consider 5-6 future cory's as well. I believe I tend to overstock the tank because I like so many types of small tetra's. I still believe I have too many fish in the tank and probably will need to reduce further.

Thank you, Steve

[MatsP]

As long as you can keep the nitrate under control, you can have quite a few fish. But the key here is to monitor the nitrate levels - if it starts going too high (above 20-30ppm), then you either need a bigger tank, bigger water changes or less fish.

--
Mats