Spring for the master test kit ($30). Also, look into a swimming pool test kit from Walmart, HD, or Lowes ($5-10). If money is an issue, get the pool kit, nitrite, & nitrate.
The Master Test Kit has ammonia, pH, nitrite, and nitrate.
The pool test kit has chlorine and pH.
The pool test kit is to test your tap water and to test your treated water to verify it is safe to use. You also need to test it for ammonia (chloramines).
Don't do lazy refills like the Python way. That is a first class ticket to dead beneficial bacteria (indicated by your ammonia readings)...among other things.
Don't discount the possibility of bacteria infections. Are there any frayed or decayed fins? Any red streaks on the fish or fins? The salt method for bacteria is a craps shoot but worth a try. Use double the normal amount of salt and lower the temperature. Raising the temperature can kill your fish. Higher temperatures will increase the metabolic rate and reproductive rate exponentially in the bacteria to the point that the fish won't stand a chance. At lower temperatures, the metabolic rate in both the fish and bacteria will be lowered but since the fish are multi-cellular, they can still fight the bacteria. Just don't lower the temp to that what will kill/stress the fish.