We think some of the fish have got fin rot. Yesterday morning we woke to a dead Molly and guppy. We went to a fish place and bought some medication for the wate4, but we have had another platy die this evening. The medication is a 3 day treatment, so far we have done 2. Is there anything else we can do as other fish aren't looking healthy either?
Check and monitor your water quality (ammonia, nitrite, nitrates) using a liquid-based test kit. You need to ensure that water quality is consistently 0 for each of ammonia and nitrite, and no more than 20 above tap water levels for nitrates.
Water quality needs to remain consistently at this level, even just prior to your scheduled water change (weekly presumably), to prevent fish succumbing to illness or dying - if any of those levels are higher, then do more frequent and/or larger water changes.