If they are dead grab the vacuum cleaner and suck them up. If they aren't dead what I've used in my house (with cats) has been Raid liquid ant bait (only about 5 bucks)
http://reviews.canadiantire.ca/9045/059 ... eviews.htmWhat I have done is take cardboard and tape. Tear the cardboard up then put the tape on the cardboard. Put a drop or two of this bait on the tape. Its a mix sugar and borax in just the right amount. You could hide these little piece of cardboard around (I don't think in the tank would be the best of ideas though). They will attract the ants and they will bring the food back. I have had extreme success with this method in my house. A word of warning though. If you set up these traps the ant problem will get much worse for about day as word spread thats there is tasty tasty food.
Every year the ants invade my shower for some reason (I really wish I knew why). I started off with maybe 30 ants in it. Put some of this stuff on the walls (its a shower it will wash off). 30 minutes later the number doubled, then tripped then probably had a few hundred ants in my shower as they all munched on this stuff. A few hours went by and the number increased still. Then by like 11pm at night there were like ten left (besides the ones that got a little too excited and got stuck in the stuff). After I cleaned the shower they didn't come back.