I am building a cooler livewell for my 15' metal boat. Looking at getting a 120qt cooler and fill it 2/3 of the way full or so, so I should have 20-25 gallons of water to work with.

I will have a recirculation pump recirculating the water and spraying from a spray bar...my question is it ok to be sealed closed?

also if I use livewell additives (please release me or whatever the additives are) and peroxide and keep the water 70-75 degrees...how often would I need to add new fresh water?

Thanks

What are you going to do to keep the water at 70-75 degrees?

In the heat of the summer use frozen 2 liter coke bottles.

Best DIY livewell I've seen

It is going to depend on the ambient temerature and the number, size, and how stressed the fish are in your livewell. Cold water will absorb more oxygen than warm, and additives will make the water absorb more oxygen. The whole thing is about absorbed oxygen in the water. A stressed bass in a livewell is going to emit amonia which depletes the oxygen even more. You're going to have to play it by ear. In the winter there may be times when you won't even have to change the water. In the summer, you may need to change the water every hour and a half. Keep it cool and put additives in the water. Plain old rock salt is what the TPWD uses for additive, but they also use pure oxygen, which is optimum. You need to err on the side of caution.