Yeah, the problem I have with this rig is that there's very little room between the rear bulkhead & the transom & I really didn't want the red, portable, 6 gallon tanks. The only other option was a custom built aluminum tank, but I didn't have the $800 that everyone wanted!
I was trying to keep as much floor space as possible & the only room I had was under the deck. A 25gal tank was possibly a bit of overkill, but I've run out of gas before & trying to get 5 or 6 miles upstream on a trolling motor with 2 or 3 guys + gear + fish is a complete nightmare! :D
I have as much weight as possible at the back of the boat, the only thing forward of center is the deck & tank & unfortunately, due to the age & design of the transom I can't go bigger on the motor. I had a 70 to start with, but it was bowing the transom out so I dropped down to a 45. It's slow (around 20 knotts unloaded) but it'll get me there!