You should take the fish out and put them somewhere else for a while and clean the entire tank to kill the ants.

Good question. I've never heard of this before. My suggestion, for what it's worth, is to take the fish out as stated in another answer. Obvioudly you have to find the reason why the ants liked your fish tank. What kind of tank is it, fresh or salt? What kind of fish food did you use, because it might of drawn the ants in the first place? What did the ants look like? Do you have ant problems elsewhere, or just with the tank?

ants go to water and plants you need to find where they are coming from and get rid of them or they will just keep coming back.you can try putting salt around your tank but the best thing would be get a exterminator to come out but that can be expensive. you can go to a farm store or wal mart and get some granules and sprinkle all over you yard and that will help.

I agree with the others totally,
except: why do u have to get rid of them?
I know many fish that will give anything 4 a good ant meal!:)
So check if they too like ants!
It'll save u from feeding them, and they'll have some eccupation! :D

I dunno but I bet the fish are happy with all the free food.