So hang on.. you want to take a betta out of a 12 gallon tank which is a fantastic size for it, and put it into something vastly too small?
I'm serious, explain the logic to me because I don't get it.
The main reason your plants are dying is because your light is broken. In which case get a new light and plenty of new plants.
Or if not, get a LOT of silk ones instead. They can't die, and are safe for betta . Problem solved.
Moving a betta from 12 gallons to a 3 gallon anything as a permanent home is utter insanity, and something 3 inches deep is even worse.
Those wall mounted tanks are just another overpriced gimmick. They're really poor "tanks" to keep anything but shrimp in - in fact, do just that, a well planted shrimp tamk with lots of mosses would look great and be far more manageable and humane than keeping a fish in it.
If he's hiding in corners and so on, not using much of the tank, you most likely don't have it planted enough, or enough hiding and resting places for him. Do you have a picture of the tank? And also the broken lid? We could help you find a way to fix it.
If the ballast does not come into contact with the water in any way, you could try using two part epoxy or even milliput to bind it back together ( leave the lid up for this to set or cure and put a mesh cover or piece of perspex as a lid to stop the fish jumping out )
Honeythorn....
Please RELAX. I only THOUGHT of the idea, hence why I posted about it asking, instead of postinf "well, I just moved my betta into a 3 gallon wall mounted tank...". After thinking and reading and seeing that tiny wall-mounted tank I looked up replacements parts for the 12 Eclipse. I do happen to agree that those tanks are gimmicks, and I don't trust them on the wall. I guess at the time, the thought of a broken tank being replaced with a free new expensive tank was getting the best of me. BUT, I thought against it. The Eclipse lids are too expensive, so with a little work, it wasnt the light that was broken, but the switch to turn it on and off. I finally managed to get it back on, and just will plug and unplug the outlet to make it go on and off.
I will show you pictures of his old set up, and I just re-did the tank with driftwood and lots of plants for him. He seems to be perkier and is going in different areas, and even checked out some of the new setup. He is still not himself though, as he is just kind of hovering by the top right corner of the tank.
But, as requested, here are the pics.
THe first picture is the old setup , but after 3 months or so, the tall plants on the left, middle AND right died...so it was kinda "eh" ish. But he swam around alot. Unfortunately, getting prepped to move, I didnt vacuum or do a water change in about 2-3 weeks...thats what must have caused the fin rot. My camera doesn't get pictures good of him, but I will try when hes out in the open tonight.
The second picture is the new setup I just re-did last night. He was exploring it some, but like I said, he just kinda hangs at the top. Good news is, he ate twice yesterday, and once this monring, but just one pellet. SO that's good. he has only one strand of long fin left, but the "rot" has seemed to stop.
THe 3rd picture is of my betta Mason before the fin rot...
Mason before the fin rot
