The salt is a pretty important part of keeping Alewives and shad pretty much year round. I use more in the summer than in the winter but it's still pretty important. It does two things well and a third OK. It helps the physical condition of the bait as far as hardening the scales and helping prevent red nose. Have you noticed much red nose on your baits? It also helps add density to the water and dense water holds dissolved oxygen better. The colder water is also more dense so that isn't as important now and the reason i don't add as much salt now as in the summer. It also helps with stress on the bait to a degree. I also add the shad saver to help with this as well but only when adding new bait to the tank. The flow through tank would have the same issues as the bubbler tank. Can you fill it and turn it off so it just stays full without the overflow? If so, I would add salt and a bubbler to it as well if it doesn't have the splash problem. Having two tanks set up has an advantage. If one goes south the other may not.

I just purchased the 30 gallon keep alive round insulated tank. It appears to be designed well with regard to water splashing out. I'll let you know how it does. It's seems to be a perfect size for where I want to put it.

If you can get buy with that .5" net without much gilling then you should go for it. A .5 will sink faster and stay open wider than a smaller mesh. Do this test the next time. After netting bait rinse the net out really well and check the water to see how many scales come off the net. If it's a bunch, then I would suggest a different net.