Aquariums: Do I redo my tank? What can I do to not have to redo it?

I would start with taking the wood out of it’s rotting away unless you planning on putting snails in and deal with the bacteria and cycle the tank for a few weeks with morning in it before putting love stock that way you can see if there’s any things in there growing that needs to get rid of

This is salvageable especially with the plants still growing. There’s a cycle established. Work with it. A gravel vac heavily over the worst areas will help. Scrape any off the glass and gravel vac it. We don’t want to shock the plants so a 50% water change should do. 

I didn’t see a filter so a small HOB filter with carbon will work to remove the particulates from the water column. For added filtration, a sponge filter is always a great addition. We want the water to move and circulate with lots of oxygen, this will help break up that algae and stagnant bacteria. Gravel vac every few days. Remove any dead or decaying plant matter and do water changes. 

Add more plants if able, even floaters help. You can scrub the wood with hot water. This will let you examine it but dissolving wood doesn’t harm anything so it can go back in if you don’t/can’t replace it.

A day or two between water changes, test the water parameters. It’s possible a partial new cycle will start and a small bacterial bloom may occur so best to test it regularly like you would a new tank but it should settle much faster than a new tank would. Once parameters are stable, you can bring that betta home and slow the waters flow so the current doesn’t blow him around. Good luck!

I’d be careful about keeping the cat in there. Mine prefers roaming the land with occasional periods of play on the aquarium…