UGF's have been in use for over 50 years, and there's nothing wrong with them. They aren't the best, but they're good for many applications. ( including ones like this )
Your main mistake was changing that much water with new water which was cold. When you change that much water at a time, it's very important to let that water sit out for a day to aerate, as water in the pipes doesn't have a good dissolved gases ratio. It also needs to warm up to match the tanks temperature. Small water changes with "raw" water are usually okay but a massive one like that is nothing but trouble.
Get that fish OUT of the little basket. That's one of the things which is keeping it from recovering.
|