I love cory cats. I started with 4 peppered cats. They were fine for 3-4 months before they slowly dropped dead one by one. No outward signs of disease or damage. The bodies had a reddish look to the bellies.
After the peppers were gone, we tried again with albinos. Same darn deal. Fine for months then one by one gone. These guys were always more active than the peppers. They were truly awesome and it killed me to see them go.
So do I try again? Give up? Different LFS? Different variety? Any advice would be greatly appreciated.
ammonia none nitrite none ( tank fully cycled for almost a year now, but I still test ammonia and nitrite to be sure)
nitrate stays at 5 ppm thanks to the tank being well planted
pwc every week, 30% minimum, plus canister and media rinsed in tank water
30 gallons, canister filter, 2 heaters, water at 76-78 F, compact fluor light 65 watts fo 10 hours
tank mates: 1 male molly and his 2 ladies, 1 male platy and his 2 ladies, 1 male guppy and his 3 ladies, 4 minnows, undetermined number of fry too small to count or sex or identify
My guess is that there is food and waste particles getting trapped into the substrate, rotting and causing the problems. Any substrate particles that are larger than sand will allow for waste and food to get trapped at lower levels. There is no oxygen much below the surface of the substrate. You mention that you have a pretty thick layer of Eco Complete covered by a layer of small gravel. This is great for plants but not so much for your Corys.
Most people with heavily planted tanks don’t vacuum the gravel so as not to disturb the roots of the plants or don’t vacuum the entire tank (leaving the areas with plants undisturbed).
That means that there is anaerobic bacteria producing gas bubbles in the substrate. While not terribly dangerous in a healthy aquarium that is well maintained, Corys like to dig through this substrate. It is this digging that can release some of these bubbles which could be ingested by the Cory causing death.
The rest of your fish are fine because they are not in contact with these gas bubble at the bottom of the tank.
You can test to see if you do have a problem with these gas bubbles by taking an item such as a table knife or something similar and poking it into the substrate. Move it back and forth a little and doing this in several areas. If you get bubbles that come out of the substrate then you have a problem. If you don’t have bubbles then you don’t have a problem and I am wrong with my diagnosis of what is causing their deaths.
I wouldn’t give up. However, you may want to change you tank substrate a little and you may want to buy Ian Fuller's book "Breeding Coridoridane Catfish." These are great little fish and definitely add to your aquarium of mostly mid and top level fish.
No bubbles detected. I've found trumpet snails in there on occasion...hitchhikers. Shouldn't they assist with stirring up the substrate?
Additionally, I do vac deep into the gravel. I have a smaller nozzle on the siphon and try to plant in clumps spaced so the nozzle can fit in between. I still sometimes uproot a stem plant or two but they are easy enough to put back.
Ugh. This is so frustrating. The kids want more cories, but I won't unless I am convinced I am not causing the die offs somehow.
Thanks for your detailed post on bubbles. I will double check for those just in case I missed them the first time I poked around in there.
What are the water parameters, i.e. pH, GH etc. I might have no bearing at all unless there are extremes.
Also what is/was your feeding regime.
Both species you mention are considered a "Tough as old boots".
I always recommend using a fine sand substrate, but that is difficult if your preferences are for plants. Corys are filter feeders in the main and naturally take in mouth fulls of sand and filter whatever food particles they find, ejecting the sand out through the base of the gills covers.
I think that there is possibly a food issue. Firstly although the livebearers are herbivores Corys are not,so vegetable based foods are not going to give them the correct nourishment they need. Again algae wafers are ok for the livebearers but not for the Corys. Not sure about the Hikari catfish food, I have never used it and would advise looking at the ingredients. Personally I would be looking at using sinking tablet foods designed for omnivores, such as Tetra 'Tabimin' or something similar, Spectrum and Omega do pretty good sinking foods.
I would certainly check the substrate for gas pockets and start a gravel cleaning regime, vegetable matter breaks down very quickly and once uneaten particles are deep in the substrate they will deteriorate and rapidly produce anaerobic pockets.