Hey stevenluther .... which tanks? Phiniacs?
Phiniac regular tank = regular sized Boges or generic carto
Phiniac Tall-boy tank = Boge XXLs or phlanged DCCs (maybe 1/8-1/4 inch out the top or bottom)
Phiniac XLs = Kanger XL or Resurrectors
Of course if you use a longer carto in the tank, punch it high, and leave the extra out the bottom, you don/won't need an extension.
You can see here a guide -
Tank Sizes & Cartos - Phiniac Rugged Pyrex Glass Tanks
Most tanks that I know of will run off any carto, the length of the carto is the variable you deal with.