So you had a dead pleco today.... a platy had Columnaris 3 days ago.... and you suspect that you've dealt the Myco recently?
If that's the case, I would buy all new equipment for your new VT today. He should have his own brand new set of everything.
(New tank, bucket, siphon, heater, filter (if you use one), decorations, gravel, etc. Everything should be new, and have never come in contact with your other tanks.)
The alternative would be to spend some time determining why your other fish are becoming sick, and solve the problem prior to getting a new fish....