Well, long story short, after using a DIY yeast CO2 system on what used to be my 55 gallon planted tank (and is now my 55 gallon non-planted but full of fungus tank), I ended up having some yeast get into the aquarium and it started a nightmare from which I do not seem to wake. 
I posted a very long version of this story elsewhere, but suffice it to say, after trying everything I could think in the tank, then tearing down the system and disinfecting everything and/or replacing things that couldn't be disinfected, I had the yeasty fungus come back due to it hitching a ride on my fish (I think some of it got caught on a fin). I left it be for awhile beyond regular scrubbing to get the worst of it off the glass and driftwood, as it wasn't harming the fish any and I couldn't work up the motivation to try ripping it apart again, but I have a couple new fish coming this week, so in anticipation of this I decided it would be best to try and get rid of the yeast once and for all (if such a miracle is possible.) I haven't seen much info on this, beyond other people throwing up their hands in frustration, so I thought I'd post everything I do to try and fix it and what sort of results I have.
First, a bit of info I've already learned:
Don't ever let yeast anywhere near your tank. Not saying DIY yeast CO2 can't work great... if set up and maintained properly they are a nice, inexpensive alternative. But lots of things can go wrong, so be careful, and make sure you have a bubble counter on that sucker.
The fungus in my tank tended to mutate in appearance: initially it was sort like flowy bits of opaque, beige, semi-gelatinous.. stuff. (Texture just like that culture found in Kombucha drinks.) Then as it grew on the driftwood it was like a thick fuzz, varying in shade from near-white to almost yellowish. Finally it appeared on the glass looking very similar to a light brown diatom algae.
Do not, under any circumstances, EVER use vinegar to clean tank decorations if you have yeast in the tank. I found this out the hard way. I dislike using bleach unless I absolutely have to, so I tend to use vinegar and/or hot water to scrub off built-on gunk. Anyway, apparently yeast LOVES vinegar. Only hours after placing those decorations back in the tank, they had fungus on them, and it absolutely exploded in the tank after that. Baking Soda has the opposite effect, but I have read that it doesn't kill the yeast, merely slows down its growth rate.
Anti-fungal medications don't have much affect on yeast growth. I tried using some after removing the fish from the main tank, just to see if it would help, and it only had an impact after I tripled the recommended safe dosage, which means it would never be safe to use with fish in the aquarium. It never killed all of the fungus either, only caused it to disintegrate some.
All right, now for the plan:
Set up and cycle 20 gallon quarantine tank (already done)
Move fish into quarantine tank, giving them a dip in Methylene Blue before completing the transfer to try and avoid yeast transference (fish have none on them, but last time they were healthy too and it seems the yeast still managed to transfer over, so maybe this will help.)
Tear down 55 gallon and soak everything but driftwood in a 10% bleach solution/scrub down tank with bleach as well, rinse, dechlorinate, air-dry, etc. Boil driftwood very thoroughly.
Re-set 55 gallon completely empty, with only filter and heater, cycle, watch for any reappearance of yeast. Meanwhile, watch Q tank to make sure the yeast doesn't show up there either.
If all goes well, re-introduce fish back into main tank. Wait awhile and make sure yeast doesn't show up. Put all of the driftwood into the quarantine tank after fish are removed and watch carefully for at least a week or two to see if there are signs of any growth. (This is the most likely place for it to show up if I didn't get rid of it... yeast loves the driftwood.)
Only after it seems certain that the yeast isn't coming back, put new gravel and the decorations/etc back into the tank. Wait yet again. Watch. If everything is good in the main tank and the driftwood is yeast-free, put the wood back in the tank. Eventually, re-plant.
Alright guys, cross your fingers for me. I'll keep you posted as to how things are coming along.
I posted a very long version of this story elsewhere, but suffice it to say, after trying everything I could think in the tank, then tearing down the system and disinfecting everything and/or replacing things that couldn't be disinfected, I had the yeasty fungus come back due to it hitching a ride on my fish (I think some of it got caught on a fin). I left it be for awhile beyond regular scrubbing to get the worst of it off the glass and driftwood, as it wasn't harming the fish any and I couldn't work up the motivation to try ripping it apart again, but I have a couple new fish coming this week, so in anticipation of this I decided it would be best to try and get rid of the yeast once and for all (if such a miracle is possible.) I haven't seen much info on this, beyond other people throwing up their hands in frustration, so I thought I'd post everything I do to try and fix it and what sort of results I have.First, a bit of info I've already learned:
Don't ever let yeast anywhere near your tank. Not saying DIY yeast CO2 can't work great... if set up and maintained properly they are a nice, inexpensive alternative. But lots of things can go wrong, so be careful, and make sure you have a bubble counter on that sucker.
The fungus in my tank tended to mutate in appearance: initially it was sort like flowy bits of opaque, beige, semi-gelatinous.. stuff. (Texture just like that culture found in Kombucha drinks.) Then as it grew on the driftwood it was like a thick fuzz, varying in shade from near-white to almost yellowish. Finally it appeared on the glass looking very similar to a light brown diatom algae.
Do not, under any circumstances, EVER use vinegar to clean tank decorations if you have yeast in the tank. I found this out the hard way. I dislike using bleach unless I absolutely have to, so I tend to use vinegar and/or hot water to scrub off built-on gunk. Anyway, apparently yeast LOVES vinegar. Only hours after placing those decorations back in the tank, they had fungus on them, and it absolutely exploded in the tank after that. Baking Soda has the opposite effect, but I have read that it doesn't kill the yeast, merely slows down its growth rate.
Anti-fungal medications don't have much affect on yeast growth. I tried using some after removing the fish from the main tank, just to see if it would help, and it only had an impact after I tripled the recommended safe dosage, which means it would never be safe to use with fish in the aquarium. It never killed all of the fungus either, only caused it to disintegrate some.
All right, now for the plan:
Set up and cycle 20 gallon quarantine tank (already done)
Move fish into quarantine tank, giving them a dip in Methylene Blue before completing the transfer to try and avoid yeast transference (fish have none on them, but last time they were healthy too and it seems the yeast still managed to transfer over, so maybe this will help.)
Tear down 55 gallon and soak everything but driftwood in a 10% bleach solution/scrub down tank with bleach as well, rinse, dechlorinate, air-dry, etc. Boil driftwood very thoroughly.
Re-set 55 gallon completely empty, with only filter and heater, cycle, watch for any reappearance of yeast. Meanwhile, watch Q tank to make sure the yeast doesn't show up there either.
If all goes well, re-introduce fish back into main tank. Wait awhile and make sure yeast doesn't show up. Put all of the driftwood into the quarantine tank after fish are removed and watch carefully for at least a week or two to see if there are signs of any growth. (This is the most likely place for it to show up if I didn't get rid of it... yeast loves the driftwood.)
Only after it seems certain that the yeast isn't coming back, put new gravel and the decorations/etc back into the tank. Wait yet again. Watch. If everything is good in the main tank and the driftwood is yeast-free, put the wood back in the tank. Eventually, re-plant.
Alright guys, cross your fingers for me.
I'll keep you posted as to how things are coming along.