# Fish (Bottom-dwellers) Dying



## dwalstad (Apr 14, 2006)

Folks, here's an interesting letter I just received that I wanted to post for comments and to help others:

I hope you can help me. One of my aquariums is a small 20 litres one specifically set up for a gorgeous betta. It is heavily planted although it is not set up as a natural aquarium - neither it is a high tech one. I have a layer of JBL Florapol as a fertile layer with about two inches of small (1-1.5 mm) gravel, and an occasional doze of CO2 (from a basic device with a "dome" dispenser and a bottle with CO2 where you need to inject the CO2 manually). The plants are clearly thriving or at least are O.K. I have Blyxa Japonica, green Cabomba, Cryptocoryne Wendtii, Cyperus Helferi and some Elodea just floating in the tank. My betta is doing really fine and I decided to add some "room mates" for him. The first addition was two otos, a freshwater nerite and four corys habrosis. One of the corys died within a few hours, and the rest gradually deteriorated and eventually died. The symptoms were like they suddenly had a swim bladder problems. I spoke to the shop I bought them from and they suggested it was something bacterial and advised to treat the tank... Their stock is usually very good so I cannot blame them for anything really. I treated the tank as they advised but the corys still died. One of the otos died a week later whereas the betta was completely fine. A couple of weeks after that I bought another oto and three new corys - panda ones this time. I bought them from a different supplier - just in case. The story repeated itself but the new oto is perfectly fine and I am down to the last cory. The corys showed exactly the same symptoms as the previous lot. 
I have various water test kits (API ones) and the water is perfect whenever I test it: nitrite, ammonia - zero, etc., etc. I am suspecting some sort of toxicity now - fatal for corys. I only have tests for copper and iron and they both show zero...
Do you have any ideas what this may be? I really want some small bottom dwellers for the tank but scared to buy new fish and let them die...

(letter from N.)____________________

Dear N,

The substrate may be generating H2S (hydrogen sulfide), which is a very common substrate toxin (my book, p 133). It will kill bottom dwelling fish.

Deep gravel layers-- with or without soil-- can cause problems. I recommend a gravel layer no more than 1" thick. Make it as thin as you can. The deeper the gravel, the greater the chance the bottom soil layer will go anaerobic and generate H2S. Also, the soil you have may contain some sulfate fertilizers that are contributing to the problem.

I would try to remove as much of the gravel as you can (save it for another time and another tank). If you have some mechanism to circulate the water, this will help keep the gravel/soil a little more aerobic until the soil stabilizes.

Your plants may also do better with a thinner gravel layer. They don't like severely anaerobic substrates either!


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## Nat N (Jun 6, 2010)

Thanks a lot for the advice. I have already removed some of the top layer of gravel. The remaining cory is still around and if he is fine in a few days I think I can start thingking about adding some companions for him.


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## dwalstad (Apr 14, 2006)

Excellent. Good luck!


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## Emily6 (Feb 14, 2006)

I had this exact same problem but the answer was a little more obvious in my case- I had tons of gas bubbles emerging whenever the substrate was disturbed. They wreaked, which completed the picture. My substrate wasn't particularly deep- I stuck the to guidelines- but I tried poking the soil with a chopstick to free some of the gas and this seemed to help, along with moving some plants around (I think this mixed the layers more).

I also noticed my Malaysian trumpet snails were NOT affected by this but my common pond snails all bit it. /


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