[mike31154]

You should be able to look inside your panel and tell whether your 20A circuit breakers have a larger gauge wire than the 15s. Unless the wiring is very old, the specs are generally stamped on the insulation. As mentioned, 20A circuits need to be wired with 12 gauge wire to comply with code and keep you safe. If it's 12 gauge at the breaker, then it's probably safe to assume that wire run is all 12 gauge. Should be able to tell the wire gauge at the receptacle as well, by pulling it out. Just make sure the breaker is off before you yank out any receptacle on that circuit.

I'd recommend two 15 amp circuits for your tank (GFCI protected) vice one 20 amp circuit. That will give you 30 total and some redundancy for GFCI nuisance trips. Next time you're in a reno store, look for the book "Electrical Code Simplified". Should be around $12 to $15 and is well worth the money. Lots of great info and pretty understandable for most folks.