I don't have any suggestions on where you can find a cheap tank but you might consider investing in a large tub in the mean time. Large Rubbermade tubs or cattle troughs are much cheaper then aquariums and make great indoor ponds or short term housing. However, if you cannot afford a larger tank you should not have boughten more fish, that was a big mistake but I think you already knew that. Eventually you will need a 55-75g for the Midas and Butti (not together I might add). Sounds like the Midas already needs it. The convicts will require a 30g and the Jewel a 20g. The Jack Dempsey will need at least 40g. You can get them seperate tanks or combine them but make sure you meet the needs of ALL your fish.