It is hard to say if it is fungus or not...the spider web description make me think it is mucus hyperproduction or slime disease(costiasis) which is a parasitic protozoan-but usually they will flash, become lethargic, stop eating
Mucus hyperproduction-this is usually related to an irritant in the water or water quality issue-overdosing or misuse of medications-mixing medications that irritate the skin....too much aquarium salt can cause this especially if you mix other chemicals/medication with it
With Flex-the fish will usually have a big change in behavior, lethargic, stop eating, hiding, open wounds sometimes-
Not saying that there is anything wrong with your dechlorinator-but to check and make sure it covers everything that could be irritating the skin causing excessive slime production to rule that out as a cause...especially heavy metals
The tank is 2.5gal correct....how long has it been set-up....filtration, live plants, all additives used, is the source water from city water supply or well water
How much and how often are the regular water changes before he got sick, water temp
How much salt did you use for the bath and how long did you leave him in the bath?