Most people don't know REALLY understand how to clean properly.
- Hot water doesn't KILL much unless it REACHES a certain temperature.
From what I know, to sanitize with HOT WATER you need the following temperatures & need to expose the ITEM to this degree of water for this specific amount of time.
- (185°F) for 15 minutes
- (176°F) for 20 minutes
Most acrylic tanks & decorations, accessories, I highly doubt can be subjected to a hot water soaking for that long without some sort of damage/warping from the heat.
That's the reason WHY if I'm cleaning for germs, I spray with a 2-5% solution of bleach & +90% water, leave the tank out in the sun for about a week, spray the inside with water every day.
Bring the tank outside, leave it out in direct sun. Approximately 2 days out in the sun should be sufficient, but I'd rather leave the tank out to bake longer just in case there's a little bleach residue left that hasn't neutralized.
You mentioned soaking your 3.5 gallon in vinegar? Do you have a picture of the bottle of vinegar that you used?
The largest container I have of distilled white vinegar is 1 gallon.
I'm pretty sure I've NEVER seen a 5 gallon jug of vinegar. Even @ Costco.
- There is a high possibility that there's some sort of disease still present in your tank from what you're describing.
- This is why a lot of people will throw aquarium related things away instead of attempting to clean after a disease outbreak. They just don't know how or it's too complicated for them.