Run my bait tank without another battery on board

Just got my Super Bait Tank II and it’s awesome! If your not familiar it’s a shad/herring tank with filter and aeration. It uses a rule 500 gph pump and I will run it continuously but it’s munching my engine starting battery. I wanted to run this on my starting battery instead of my trolling motor batteries-mistake??? I thought I’d be smart and bought a deep cycle marine starting battery so I could draw it down farther than a standard starting battery. It’s a car size battery (not the bigger chassis like the big trolling motor batteries) with 690 marine cranking amps. Figured I could run the engine and charge up the battery to keep it going until I could get in for a full charge at the dock. I’ve been running a test for 3 hours with tank full and the pump on-started with a full charge and I’m already down to 12.5 V (13.4 at start) and 75% charge. I know my engine would start at 75% but I want to be able to run at least 4-5 hours without worrying about getting engine started and I am thinking the battery will be tapped if I wait that long to start engine and charge the battery with a good long boat run.
OK my other option is to run off the trolling motor system but I don’t want to prematurely drain that system. I’m running a terrova 80 lb with optima deep cycle blues. I think their rated at 925 cranking amps but I’m not sure. I’d have to pull power from one and I don’t like the idea of running one way down below 50% charge after a days fishing.
Who’s doing something similar and how are you getting by without a seperate battery?
Thanks.

I just checked the web site for Rule and they’ve got same pump in 24 volt and so I’ll order one. Problem solved.

That seems like the best solution you could hope for. It isn’t cranking amps you need to look at, it’s reserve power. I always wondered how people ran those 3-4 amp pumps for a long period of time. You can’t really charge the battery much from an outboard. They’re amp output is rated at pretty high rpm.

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That seems like the best solution you could hope for. It isn’t cranking amps you need to look at, it’s reserve power.

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Actually the amp hours is what you want to look for. Your deep cycle trolling motor batteries should be fine as long as they are not getting old. New ones would run all day as long as you are not using the trolling motor too much.

Well, I went to pick out a new battery last summer and they were rated “reserve capacity.” They’ve chosen their own convention. I think it’s the number of minutes @ 20A discharge. I guess this was a combo battery so maybe it was rated with a different scale. But anyway, I was trying to be helpful.