My platy hasn't been looking too well over the last couple of days - lying on the bottom of the tank a lot, lost weight, clamped fins. He is still swimming around at times and seems to swim well, comes up for his food in the mornings and is eating, haven't seem him gasping for air at all.

He appears to have some whitish stringy or fluffy stuff on the top between his head and fin and also by his lower fins - unsure if it's something like a fungi or worms, or looking at photos might be raised and damaged scales/fins. Eyes are clear, can't see any white or black spots anywhere.

About 3-4 weeks ago I swapped over to live plants. Two weeks ago I added 3 cory's, who are all doing well and looking good.

Going by research it looks to likely be either fungal or possibly worms. Here's some (not so great) pics of him:

Was thinking of picking up some Melafix and Praziquantel tomorrow... good place to start? How long would it take/how many treatments for him to start looking better if they were working?

Oh, and some useful info: I haven't got a test kit on me at the moment, but before the Corys were added 2 weeks ago Ammonia and Nitrite were 0, Nitrate was low. I do 25% water changes twice a week. pH is stable at 7.4 and temp 25-26. Have had a bit of an algae bloom over the last week, not sure if that's an indication of a nything?

Best place to start is do a water change. Before doing so gather a sample of water and take it with you to the shop to see if they will test it for you. Clamped fins could be ammonia or parasites, whitish stuff could be slime coat coming off from a possible ammonia spike. Clamped fins can also be parasites. In saying ammonia I would have expected the cory to be off colour too.

Algae is becoming a bit more prevalent in Auckland this week with the change of seasons and the lower sun. Algae feeds off nitrate and phosphate so testing will help with that as well.

Have a look at it at night with only a torch and see if it has velvet----looks like it is sprinkled with pepper. Hard to see much with the pics.

Unfortunately the platy died last night From what I've read and photos I've seen I believe it was internal parasites.

So since the Cory are still in there should I continue to give the tank the remaining doses of praziquantel and melafix? Wouldn't want anything left in the tank that could then infect them.

I did a 50% water change today and cleaned up the substrate as much as I could in the process.