We have been away and the electricity tripped out leaving my tank without the filter going for approx five days. I got home and couldn't even see the fish. Turned out they were hiding under a fake plant.

It's taken me ages three months) to get the tank cycled, and it only got there a month ago, so gutted.

However, after getting the filter going again, doing partial water changes daily (it's been 3 complete days we've been back) and doing daily tests it seems all is not lost. Fish are now behaving normally and swimming around as they always used to. There was a spot of ammonia to start with, but that is now back to zero, Nitrite is zero (and was on first day of test). So seems my cycle hasn't been lost. Nitrate is about 20, could be higher (I can't decide), and stays that way despite me doing a third water change yesterday. It always used to hover lower, between 10-20.

But one of the fish has developed a very small furry white spot on the leading edge of his top fin. Poor guy was probably stressed out during those five days.

So, my questions are: should I do a big water change to lower the Nitrate?

And: How do I treat the white spot? I have some Myxazin, and some Melafix (which I don't have any instructions for as someone gave me some in a pot). I also have some Sera Baktopur but that's pretty heavy duty stuff I think.

I have always received great advice here in the past, you guys taught me all about cycling the tank to keep the fishes healthy, this is my first wobble since learning all about that so just need some advice to get tank and fishes back on track.
Cheers all!

It never hurts to keep the water as pristine as possible when dealing with stressed fish. Water changes are a good thing in my opinion.

That said, if the ammonia/nitrite stays zero and the nitrates show that the cycle is working that is GREAT! I like to keep nitrates at or below 10 - but I am a fanatic.

As far as the "furry" spot - I would touch it with a bit of hydrogen peroxide. On the dorsal, you should not have a problem with getting HP into the eyes or gills (something you wish to avoid). I would take a small plastic syringe filled with a tiny bit of HP. Lift the fish out of the water, but hold it over the water. That way if it flips it falls into the water - not on the floor. Then syringe the HP over the affected spot on the dorsal fin. If you do not have a syringe, you can use a que tip, cap of HP or even a cotton ball if the fish is large enough.

Then just place the fish back into the tank. A little HP in the tank will hurt nothing, but it will sterilize the small infection/fungus that is affecting that fin. Watch it carefully.

I have found that you get much better and more immediate results from mechanical sterilization than by treating and entire tank with an antibiotic medication. If you have it, and wish to, postassium permanganate is also a great mechanical sterilizer - also an oxidizer. You should "rinse" your fish, following a topical application of PP on the fin, though, if possible.