I have recently taken over my sisters 4 gallon because it is sitting empty with nothing but snails in there. I would love to turn it into a invertebrates only tank, and I was thinking of Red Cherry Shrimp? Anyother Suggestions?? But I was wondering, I am going to get a heater, but the tank it self, the only filter type thing is a small tube running up from the undergravel filter. And my sis said that it came with just a air stone down in the tube, it is not the same diameter as my undergravel filter, but what do you think I shouled do?? I would love to have the tank, got the perfect spot, but I dont want to spend more than 10 dollars on it. And the heater doesn't Count, I plan on using one of those mini heaters, but for the filter what do you suggest?? The tank has gotten BAD before, I mean like dirty, filthy disgusting, the water is bad even when I do changes. Please help me by having a solution for a cheap under 10 dollar filter thing that will work for this. Or is the one now ok? Thanks for any help.....