Sealant

[Jeremy]

I have a fish tank that has sat empty for a while and the sealant has gone a
black colour. Whats the best way to clean this ?

Thanks

[Joseph]

Oxyclean claims to be tank safe.

I used it to clean an isolation tank. I just filled the tank, threw
in a couple of scoops and let it soak for 48 hours....

Just be sure to rinse the tank out several times, and dry well.

Joseph