well until you can manage to get a larger tank ill give you some constructive information that may be helpful in your current situation.
When you do waterchanges are you using dechlorinator (i dont think you mentioned or not)? It really is essential to dechlorinate the water. Chlorine is no good for the fish. Also, to help with the changes and to do a good cleaning of the substrate with the changes you can purchase a syphon or even just a length of plastic tubing to suck up the crud on the bottom while doing the water changes (a turkey baster is also helpful and work well in small tanks to suck up the bottom "garbage"). Also dont overfeed the fish. They could be put on an every-other day feeding schedule and dont feed too much. In a small volume of water like that the ammonia (which is produced by the fish in their waste, or any decaying material in the tank such as old food) will build up fairly rapidly. Without a filter that has been running with a source of ammonia for a while ( "cycled" filter) that ammonia will build up rather quickly. The only way to remove it without a proper filter is to do water changes. I would recommend a very large one (70-80%) and then partial (30-50%) every other day (maybe the day after they are fed). Live plants will help a little with the ammonia as they use it as a food source. But, without proper lighting in the tank the live plants will eventually die and rot adding to the ammonia problem.
Why are you using the Ich meds? I dont see any signs of ich (although the first guy looks like his fins are clamped which is a symptom of the environment not being very suitable-but likely more frequent waterchanges will sort that out.).
How do you know they are biting their tails? Have you seen it or just noticed large chunks of fin going missing-that can also be from finrot, usually caused by poor water quality.
While all this seems overwhelming right now the best thing to do in all honesty is just keep up with the waterchanges and cut back on food. The fish will not starve so dont worry about that. Keep in mind fish tanks dont absolutely NEED a filter, they just make it easier for the hobbiest so we arent constantly changing the water to get rid of ammonia and other toxins. But without a filter it is most important that you stay on top of the waterchanges!!! A syphon and a bucket will be your best friend until you can get the tanks sorted. But realistically changes on two tanks that size should take about 5 minutes total.
Just my two cents. I realise that you were probably given advice from the store and since they work there you trust what the employees are telling you. You will find as you get more into the hobby and begin understanding fishkeeping that often the advice given is not entirely accurate or correct. Well dont you for coming to the forum for some help and guidance.
Also that by only advising to rush out and get a bigger tank or get filters isnt always possible in every situation-whether it is based on location of the person or their wallet. So for now, until you save up your pennies, just stay on top of the waterchanges (purchase some tubing or a syphon and a bucket-even a large plastic tub to help with the water changes)! You would be amazed at what clean water will do for a fish's health. In the meantime check craigslist (or similar) and garage sales for new tanks. Ive found some amazing deals that way (basically GIVING the stuff away-the owners just wanted them gone really)
all the best! and Good Luck! Your boys are cute

Cheers