If you are using test strips which brand, so I can search for a chart?
The bigger tank would be fine if you finish rinsing and get it filled up. With fresh water.
You didn't say what if anything you are using on your well water to make it safe to drink, UV, chemical, - what?, or ???
Those fish are ammonia making factories!!! That 10G could kill him just on how fast he can produce it and how little water there is in there.