Yes. Novus can be used inside the tank, but make sure you rinse out the tank afterwards. I bought a 350 in horrible shape and spent 3 weeks working it with Novus inside and out. Cordless drill with a soft buffing wheel and low rpm's helped a lot.
I used novus on my 240 gallon inside and out. I just wiped it out real well with paper towel after I was done. Then I filled it up and ran it for a couple days then drained it completely. I did that one more time before I began cycling the tank and I have had no issues.