Step 1. Get yourself a new heater and 2 new thermometers - preferably an LCD type that sticks to the glass and a traditional bulb type that uses a suction cup to stay on the glass. A 50watt should do the trick and set it to 74 degrees to start. Monitor the temperature and adjust so that it maintains a temp between 74-76 degrees.
Step 2. Pick up an inexpensive gravel vacuum. These are nothing more than gravity activated syphons made of clear plastic tubes and hose. Begin getting yourself in the habit of syphoning off the debris and waste that build up on the gravel every week. Do this once a week until you have syphoned off the waste and approx. 1/3 of your water from the tank.
Using a clean bucket and some dechlorinator, refill your tank with fresh water. Do this every week without fail in order to maintain a good water quality.
Step 3. Since you are on a budget, you should take a water sample (before you do your
water change) to your local fish store and have your water tested weekly. Ask them to test your
ammonia,
nitrite and
nitrate levels and provide you with the readings for each. Keep yourself a little diary of your weekly water tests. You want to be sure that the ammonia and nitrite levels are at -0- as these chemical compounds are highly toxic to fish. The nitrate which is toxic, but nowhere near as toxic as the other two should be kept at a level less than 20. Preferrably less than 10. This is done by doing your weekly syphon and water changes.
Change only the filter media - (the charcoal filled sack) and do this about every 4-6 weeks. Do not rinse the filter media - (This could be a sponge or bio-wheel or some other sort of pad) unless the water fails to flow through it. Then, rinse it only in the water you've syphoned out to be changed and do not squeeze it dry. This is where all of your good bacteria live...You never, ever want to replace this unless it begins to disintegrate and you never want to wash it in hot water or clean tap water.
Do this and you should begin to see your fish happy again.
I'll let someone else with more experience with diseases give you advice about precautionary treatments.
After reading your tank conditions it just sounds like you haven't been able to keep up with your regular maintenance. Once you get back on track things should begin to get better. And if for any reason you find that this is too much work for you, then please give your fish to someone who might be able to provide more care for them. Please don't take that wrong, but we're all busy and we've all got financial obligations and it's sometimes hard to prioritize and in all honesty, some things are just much more important than changing your fish's water at times.....
Good luck and keep us posted.