Fish are normally ill for a reason, so just putting medicine in wont help, you need to fix the problem first.
Is everything working? Heater, filter etc?
when did you last clean the filter?
when did you last do a water change?
any fish bullying the others?
before treating i would check all working, if the filter is very dirty give it a clean (use old tank water is better than tap, the chlorine in the tap water can kill the bacteria you need in the filter.)
Do you have any water test kits, ie Nitrite or PH
PH has a tendancy to get a bit acidic which can cause problems, 1/2 a teaspoon of bicarbonate of soda diluted in a pint of water can help if this is the case.
If nitrite shows high, it indicates your filter isnt working correctly.
If you cant do the above, do a 50% water change, then treat, likely more effective.
What are the ill fishes symptoms, I may be able to help further.
Jimbob, where do I start.
OK, everything's been fine for months. Dalmatian Molly gave birth to 30 fry about two weeks ago (and a Orange Platy shed about 10 the next day!!). All the fish were fine and I was going to get a new larger tank this week.
Last Friday, I was giving the tank its usual 20% water change. Took the old water out, cleaned the filter added the usual dechlorinator and cycle solution. Went to refill and found our water had been cut of (Its since been cut off twice a day while they re-lay mains about a quarter mile away). Anyway, as soon as the water came back on, I filled and emptied about five buckets before I refilled the tank. The water looked OK in the bucket, but rather cloudy and yellowish in the tank. I didn't have time (or cycle solution) to do another change and I thought that the filter would cope with things. It did, in fact, and the tank became clear. On Saturday evening, though, I thought that the Molly mum and dad were acting a bit odd (shimmying), but thought it might just be a mating thing. On Sunday, I found one Platy dead and the Mollies were still shimmying and not going for the food as much as before. Couldn't get to the fish shop until Tuesday (due to 2yo son's illness), by which time I had lost the dad Molly. Started the 3-day treatment that day. Too late to save the Molly mum, who appeared to have a bit of a white spot near her dorsal fin.
The fish are looking a bit better, but the two small adult mollies are still shimmying a bit. The fry, the 4 blue tetras and the 4 neon tetras appear completely unaffected and are swimming and eating fine. I've tested the PH, ammonia and nitrate levels and they're all fine.
The water supply has gone off twice again today. Looks a bit cloudy in a glass tumbler. Could there be a connection between teh water outages and the fish disease? Dunno, but I don't think I've finished pulling the bodies out yet.