Really Need Help!!!

[Enantiopus_Melanogenys]

Hello! Everyone
I am a really beginner of Catfish and I really love Corydoras very much. I have some Corydoras Wild Caught in my tanks such as Corydoras Duplicareus, Corydoras Sterbai, Corydoras Adolfoi and Brochis Splendens. I used to keep them in my 55 gallons tank with all my Wild Cory and Wild Brochis, and they live well together even though they are not really 100% happy as I thought. Because my city tap water, San Anotonio, is very hard, so alkaline and I know that all Aspidoras, Corydoras and Brochis don't really like very hard or so alkaline water and some of them can cause to dead. So, one day, I thought I have to make them happy as they are from their home, amazon river, so I have bought a reverse omosis machine, Coralife Pure-Flo II Ro Unites, to make my tap city water, San Antonio, is clear as possible as I can and become so clean for all my catfish, and the tap water become acidic about 4 or 5 acidic from 18 or 20 akaline of tap water in my house. The KH Cabonate Hardness is down from 18 or 20 to 4 or 5, but GH is still the same. I don't know why. I don't really know between KH and GH, which one is more important for all catfish from amazon river. If someone really know, please help me and how I can change and get them better. Oh! after I use RO water from tap water all my corydoras Wild died everyday or every other day one by one or two. I feel so disappointed very much because I think it will help my catfish get much better health, but it makes them died day by day. Please! I really need help before all of them gone forever. Before I used only tap water, they died little by little, but not died every other day as now. I really don't know what I should do now, or after I use RO water, I have to put some vitamin to make them feel so good as their home, and what kind of vitamin. Please! help I am waiting for the answer everyday. I have tried to read all info from internet, but it still don't tell exactly about my problem, so please! help. Thank you so much.

Bunthid or Max

[NEONCORY]

First, I don't know how fast you changed your water from hard water to soft water. The fish can't adjust to the new water that quick even the new water is generally speaking more suited or more natural habitat like water.
Second, although I don't know where you did get the fish from. Either from local store, mail order or some importer since you mentioned the "wild caught". You have to know your source of the fish and need to find out what water parameter they were kept. Because I suspect they were kept in the local tap water if they were from your local store although they were wild caught. And store acclimate the fish to the local tap water slowly over period of weeks or even months. I don't know exactly how much of ph you can safely move in a week. But every number change would be 10 times acidic or alkaline. For example, if you change ph 8 to ph 7, ph7 is 10 times more acidic than ph 8.
I know you meant well, try to change the water to more natural water, more close to their original habitat but they may well be adjusted slowly to your local water already. Although your local water is hard alkaline water far from the water of their origin and it probably not the best water for them in a long run IMO, they are now used to it since they are living in that water parameter for a while. The store took time to acclimate to that and you have to take even longer time to re-acclimate them if you choose to change the water parameter again.
I personally don't know what is the optimal condition for the each Corydoras you mentioned. Yes, most prefer the acidic side but there are some prefer little alkaline. Same goes with temperature and hardness of water, so you need to research. And what is the best for all of your Corys. But if their preference is far apart, you probably have to get another tank or pick one specie over another and return them to store or something. Since your tap water is hard alkaline, it probably make more sense try to pick more alkaline tolerent to nuetural prefered species. Instead of try to keep the acidic water prefered species. There are some probably not many but some out there. This would be your long term solution.

Here is your short term solution. Do the water test of the water you are currently keeping your Corys. And post the result as well as how quick you changed the water parameter and how long you have been keeping the Corys in your new water(RO water). You need to post both water parameters(ph, gh, kh and temperature) before and after. And I am not quite sure when you wrote your local water is like 18~20 alkaline and 4~5 acidic you do want to change. Those numbers are extreme. Most Corys llive in 6~8 in their habitat, I think.
Anyway, those information probably need for some expert to advice what is the best solution for your situation. Frankly I don't know if you need to move the water parameter toward to alkaline slowly again or not since it sounds like you changed the water parameter too quick too soon. Sorry I can't give you definate answer but I think you need to enclose more information before someone can give you good advice/solution.

[wrasse]

Its understandable that you want to provide ideal water conditions for your corydoras, similar to their natural habitat. But these cats are quite tough cookies. Over 20 years ago my first job was working for my LFS in Warwickshire, UK, the tapwater was and I believe still is very hard and alkaline around ph8. The owners of the shop specialised and also bred corydoras. When C. Panda first appeared in the hobby, they were among the first to breed it and they did not adjust the chemistry of the water, even though they were either wild of first generation fish. They also bred S Barbatus and supplied the shop with bronze and peppered corys.
So go easy when you start adjusting your tapwater with RO water. Use your tapwater to dilute your RO, maybe 50/50 at first. Ask your LFS what they do, if anything, to adjust the tapwater. And remember that your filter bacteria also has to cope with the changes.

[MatsP]

If San Antonia water is anywhere similar to water further north (Austin general area), then it's close to pH 8 and very hard. I think that's the case.

Fish don't really care about GH, KH or pH, as long as the values are reasonably stable.

For breeding, it is SOMETIMES important to have the right values (or change the values in the right direction, as the change may be more important than the actual direct values).

Also, GH, KH are two different ways to measure mineral content in the water.

KH has a bearing on the stability of the tank - it shows the "buffering capacity of the water", which essenitally means how easy or difficult it is for the tank to have a pH crash - a pH crash is caused by low buffering capacity and a buildup of acidity in the tank, causing the tank to suddenly lower the pH - this can kill fish if you're not lucky enough to notice it early on [and it can happen really quickly once the buffering capacity is zero].

GH is just a measure of the "calcium concentration" in the water. Depending on the test method (which may vary from one test-kit to another, and certainly in a lab there may be different tests to choose from), it may also measure other minerals "as calcium" (e.g. magnesium with is chemically similar to calcium). From the fishs' standpoint, it doesn't matter if the minerals are calcium or something else [as long as it's not poisonous of course], but what DOES matter is the conductivity of the water (that is, the ability to carry electrical current). Some of the "sensing mechanisms" on the fish are sensing weak electrical currents, and different levels of conductivity will affect this sense. KH, salinity and a long list of other "mineral contributros" affect the conductivity.

Sudden changes in conductivity can severly affect the fish.

For the fish, the pH needs to be within a certain range, which varies from species to species, but 6-8 something is normally accepted by most fish (unless they come from special regions that are significantly different from that - such as the ones from highly alkaline "rift-lakes" in Africa, or acidic black-water rivers in South America to give two opposite examples). Sudden changes in pH is much worse than a stable pH in the "wrong end of the tolerable spectrum".

Obviously, replacing large amounts of hard alkaline water with pure RO water would suddently change both the pH, the buffering capacity and the conductivity, all at once. This needs to be done fairly slowly to make sure the fish are OK.

Also, introducing fish that have previously been in hard alkaline water to soft acidic water would have to be done slowly, at least over a 24 hour period.

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Mats