Thanks for all the advice on this. Good to have someone else looking over my shoulder making sure to at least warn me when I'm doing something I shouldn't.
Yes, we're just using these as transfer pumps. One for each direction since they all seem to have a check valve built in, the only reversible ones I can find are pretty expensive.
14ga seems overkill for 5A. I use 16ga for almost everything. Will take a closer look at the ABS wires for sure, I like the idea of just using them for power and then grounding the other terminal of the pumps. Will probably see what other wires are running back there we're no longer using too.
On the topic of wiring, any suggestions on a relay/fuse good for 80+ amps? Want to put both the pusher fans on the secondary fan circuit. Figure I'll switch the big relay I add with the stock wiring/relay. Those fans pull almost 30A each (over 30A when submerged in water apparently).
Planning on putting both the puller fans on the primary fan circuit, they are small and don't draw as much so I should be good there. Currently have one of each on each circuit on 30A fuses.