After reading your post regarding this problem you are having with your 20 gallon tank, I came to a few conclusions, firstly you are definately over medicating the tank, if you have used a medication, a certain amount of time has to have elapsed before trying another one, also an activated carbon sponge should clear away unused medication fast! I would recommed that you empty that tank completely and transfer all your fish to a tank that is untreated and has aged water in it at about 76 - 78F, I find that between that most fish like being kept at that temperature. I have a 15 gallon tank and I have quite a number of fish in it, including Cardinals, Hatchet fish rummynose tetras, glass cat, and corydoras and Banjo Catfish. The best thing to do with any tank especially one of about 20 gallons, is to take out 20% of the water every fortnight, that has always worked well for me. The fish dont like large water changes, just little and often. By the way clown loaches should not really be kept with very small fish, they have a tendency to try and eat them while they are swimming, and I feel that is why a caudal fin as dissapeared off one of your little fish. I kept clown fish with guppies at one time, when I first started out in the hobby. Very bad move I saw the clown fish grabbing their tails at night and trying to kill them, the reason I was investigating this was because I had bought quite a few male guppies and they kept dying one by one being picked off. By the way never keep Red eyed tetras with guppies either as the Red eyes will attack them at night. Another good thing that you may use in your tank, is a nitrate remover sachet which removes excess nitrates, it works very well and can be added to the filter or under an undergrave filter. It can be recharged too, quite cheaply. I really do hope that you get your problem sorted out soon. Also I would like to just add that Clown fish are nice fish to keep and if kept in a large tank 4 or five together they will be very happy. They are not aggresive to most fish it just seems to be guppies and maybe other small fish. I did actually keep about 4 clown loaches in my 15 gallon tank, and they were fine and happy, and used to hide underneath some bogwood in the middle of the tank. I only moved them out because of the guppy situation. Now I only have two and they reside in a 39 gallon tank.