Join Date: Jun 2008
Location: Orange County, CA
You will probably have an initial ammonia spike, but how long it last depends upon how much plant volume you have and if how much beneficial bacteria you have in your filter and possibly substrate. I only mention the latter because some people have mentioned keeping bags of old substrate in new tanks temporarily, but I don't know its effectiveness. You would have to use testing kits to verify when everything is ok.
I put my fish back into the tank within 6 hours of changing to a soil substrate. I have a lot of plants though, and kept my filter running in my temporary tank. I also aired out the soil for 12 hours. The fish seemed fine. I wouldn't put them in there immediately though.