I have the Perma Air Pad 20 tank and I think it's bad. Local retailers sell other name brands. How do I figure which one to buy?

Doesn't make much difference, they are all about the same. Just get the same size and read installation instructions.

You have a 20 gallon diaphragm tank. Might not be a bad idea to consider going to a thirty gallon tank as it results in longer pump cycles which is a good thing.

Might add that your tank came with a 5 year warranty. Probably is older than that, but just thought I'd mention it.

http://www.wstanks.com/permatank/pdfs/PPMBR00108PermaAirPumpTank.pdf