I would go with the 75. It is what you have been thinking and planning. The idea that a tank is "to big" seems just crazy to me. These things come from the ocean for crying out load. It seems to that if you had a system big enough,that seahorses could supplement if not feed solely off of the pods in the tank. I am not sure that a 75 gallon tank is big enough to do that but sure lends itself to it better then a 65. I have a 75 and it really is not that big when you get filled up. From what I have read in the forums, it sounds like seahorses are pretty smart critters. Go with you gut and stick with the 75 IMO.