I think your idea will work. It's worth a shot anyways.
I've been looking at doing a cold water tank too and collecting specimens myself. The cooling part of this is the expensive/tricky part. I agree with the duty cycle comment. I've heard many stories of these DIY systems using fridges, etc and them burning out. If you design a fridge system right it shouldn't burn the thing out but you still have the risk. I've been toying around with some different ideas and seeing what works before I make the plunge.
I've used a Peltier device on a HUGE heat sink with a fan on the hot side and a ceramic post on the cold side which I'd stick into the water like a heater. With this I can hit -35C in air and about 0C in a tub of water (with some ice formation) though I don't know if it could handle a whole 15G yet. Yes, I know that the ocean isn't that cold, btw.
One thing I am trying right now is to use a heat exchanger plumbed into my taps that will cool incoming tank water. Upside, it's cheap. Downside, you're subject to the temperature of your city's water. If you're doing tide pool stuff, this should work; when diving, I typically log the shallow temperatures around 10C in the winter and up to 15C in the summer. You need about 4 - 7C for deepwater stuff.
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