Awwwh, sorry to hear about redfish. :(
When I do 100% water changes:
1) Cup the fish. I use their petstore cups but any cup or mug will do. cover it so they can't jump out
2) remove decor
3) empty all the water
4) add a little water back in - enough to cover the gravel.
5) stick hand in gravel and move it around - to get out debris/poop
6) empty water
7) repeate #5 and 6
8) add back decor
9) fill tank
10) add conditioner
11) put cup into tank
12) add in a little fresh water into cup
13) wait 15-20 minutes, add more water into cup
14) wait 15 more minutes
15) release fish
I refill it with lukewarm water and if you check what temp it is before you dump out the water, try and refill using a temp that is very similar. Not cold or hot but luke warm seems to be a good place to be
if it IS a fungus - Quick Cure works wonders. The company that made it was bough out by Mardel so the packaging may not look the same anymore. You should be able to find it at Walmart if they have a fish section...
I have never used this one:
If you have access to petsmart or a local shop that sells fish medications, it may be a good idea to pick up an antibacterial med, just in case. Some bacterial infections like columnaris can kill in just a few hours so having something like API's Furan 2 is a good idea.