You should know your water parameters reasonably well as they relate directly to how much you look after your tank: overfeeding, water/changes, etc.
Have to be honest and say there's something decently wrong with either your LFS choice or your tank, because you need some pretty stressed fish in bad water for them to have both a mouth fungus and ich. Other helpful information that you should give us:
- How long has the tank been running and what size is it? (volume and measurements)
- What filtration? (Gph/Lpg is most useful)
- What fish are in it? Which are affected and which aren't/they all are?
Are you using tap or tank water, are you using water conditioner, how long have your fish been sick, any information you can think of that will help us identify what a possible cause is would definitely help, the information you've given us so far is terrible.