I live in East Texas and it gets pretty hot here so, Atlantic Green Herring like I got in the Netherlands are not something I can get locally, at least fresh or alive. As I think about cold-water fish, I also think about Trout.

So assuming I built a small building with ~300-gallon tanks, is raising Herring or Trout practical? What would I be looking at to cool the water in Summer? I assume as long as it doesn't freeze solid in Winter it won't be a problem with brief periods of freezing weather with limited thermal warming from lights, pumps, etc.

Is raising cold-water fish like this impractical due to cost or complexity?

TIA,
Sid

p.s. I realize this might not be the best forum for this type of question so, a pointer to a more focused forum for a cold water closed ecosystems would be appreciated!

In the summer, aerate the water that comes from a well to pump into the tank. As long as your well water is below 70°F and your O2 levels are above 5 mg/l (7 mg/l would be better) they will be fine.

I am assuming your well water is around 65°F

As for being practical, that is only a decision you can make - we have no idea what size you want to raise them to and why you want to raise them.

If the well water is 65 degrees, and you have to keep things under 70 degrees F, that's not a lot of margin considering what the ambient temperature will be in the structure where you house these tanks.

You'll also have to do some kind aeration, so the consideration of what temperature the 'air' is will play a factor too, I think.

I believe all well water is essentially devoid of dissolved O2, so getting that water 'usable' may have a temperature change consideration.

Sunil, when Cecil was using the packed column type of aerator, he saw a reduction in well water temps during the summer due to evaporation cooling the water running through the packed column. Cecil can keep the water in his 1/10 acre pond in the low 60's with 54°F well water. If the water is too warm in the 300gal tank, the OP could always plumb in a chiller to cool the water more.

Understood, but Cecil wouldn't have had to deal with ambient heat in East Texas. Even in an indoor facility, it's going to be costly to air condition down low enough in temperature in what may be a steel building or wood barn.

Keep in mind the problems associated with brining in a species that is non-native to a state. Even if kept in a closed system you may have trouble stocking them in your pond or other's. I would stick to species allowed in the state.
Transporting non-native species across a state line can get one in big trouble.

Can herring live in fresh water? If not, there's a lot of salt to add for a pump and dump system.

I wondered the same and did a quick search. Apparently, there's plenty of herring species that live in fresh water.