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I have a red eared slider who I've had for the last 25 years (So he's about 27 years old). His top shell is about 8" and his bottom shell is about 7".

His tank (48" x 18" x 21") which I've had for the past 12 years, just started to leak.

I know I could repair the leak, but I am wondering if I'd be better off getting him a new tank instead. Should I be getting him a larger size, and if so, what size is best? Wider/taller?

Your tank is about 80 gallons, which is a good size for him but bigger tanks are always better. Ideally, wider and longer are preferable over height. You would also need a new stand and the costs could add up. Have you resealed his tank before?

I did, a few years back. But that was a smaller leak.

I figured that wider was likely the ideal. What do you recommend for wider tank sizes? How wide is ideal? I may or may not be able to afford it/fit it in my house, but i want to know my options.

Also- what types of basking platforms are recommended for Wider tanks?

Also- i’m not creative/handy, but others in my family are. My turtle is a master escape artist.

Most tanks are "tall" like yours, so wider tanks will be harder to find pre-built and I'm not sure if they have any sizes to be considered standard. One site has 17" as a height on their wide tanks, so they have 95 gallons @ 48 x 24 x 17, 125 gallons @ 72 x 24 x 17, etc. There are also stock tanks if you don't mind losing viewing areas and they are much cheaper than aquariums.

So, here is what i ended up going with!! I still have to build a better platform for sunning

Looks nice! Some basic carpentry can give you a huge amount of options. There are a couple of discussions about this right now on the facebook forum and there are some older examples on the forum.