Dried potatoes for a tortoises "no" yours is an aquatic turtles and the requirements are different. Did you look under "nutrition" on this forums main page for ideas on veggies and pellets. To me even good turtle pellets are a little high with the protein side. Several in the 38-40% range. I have changed to Hikari wheat-germ formula its a "KOI" food ( only 32% protein) Koi and goldfish are vegetarians. I found it help's a little more with turtles that need a little help with the shells especially when shedding. There's some documentation on that and my real time use proven to me. These plus a well mixed pellet and veggie's , balanced and nutritious diet goes a long way on his health ! This are things you pick up on in years , even "me " went through a learning curve in the beginning ! I'm an old man , us old timers never had a forum for help , it was all trial and error back then.
The 90 gallon Steve recomenmended is a good size for him better than the 60 gallon you thought earlier. Males normally stay smaller than females but some could get 9" and you still can make the 90 gallon work filled with an "ATBA" if so. That will depend on his "DNA". Look for used may find one $100. Even stock tank that size are cheaper . Thats what I use and I think it look's good. Can do so much more with one than a narrow glass tank ! My Piggley love his !!! Look at Piggley in photo gallery for pic's , may get some ideas too.
Tweaking the UVB , the zoo med is a so so bulb but OK for now // read on the box for placement very important , yours looks too high and not surving much a purpose. Output of useable UVB rays on that UVB is not far. These are a few popular type and the differences // A better UVb bulb goes a very long way to shell health :
https://www.bing.com/videos/search?q=UV ... ORM=VRDGARThermometer you have is inauccurate , I have a draw full of that type even inaccurate digital ones , just look at all the different reads in the store when you look at them. . Infrared type is great ( very accurate) and can be used on everything needing only one. Watch for sale's and use price tracker for better price. Watch Black Friday sales , some prices are good but not all on sale is a real sale.
Basking dock is "ok" for your frist one that 12" may only have one or two different temp ranges on it. A larger one will offer more range of temp so your turtle can pick what it truely needs and when. Also a timer help's keep a schedule well , turtles like schedules ! On 10-12 hrs day ( make's it avaliable when he wants it and or needs it ) and off at night to sleep well , they are just like us. I save all my old stuff I use chop sticks and put two old basking docks together and make my own larger one to save money. Looking at other peoples basking docks may help you being creative and use materials around the house to save money. For $1 a basket from the dollar store can make a large basking area just get creative. Google basking areas and click on images for ideas on how. Also look under basking dock photo's for ideas on this forum. Tweaking things will help your turtle's enviroment inturn helping it to shed better. If diet is corrected has a chance to correct it's shell , tweaking it's home will also help but some conditions are permanent . Will still take time , they normally only shed once a year when "healthy" then some in less health maybe only every 15 months to two years. This type tweaking will help you turtle with all it has gone through with those other people and he will love you more for all in the end showing you all his personality's which is great ! If you picked the turtle or he picked you they are not an easy pet or a cheap one with the initial setup . There were ways to keep cost down and still are ways with doing more homework now , DIY helps , being creative too. Most all beginners do the " learning curve thing".
One big thing that helps "me" is a notebook , have some over 55 years old. I'm an old man and computer's are not my thing ! I kept track of every thing I ever tried and done with turtles. I can go back and see what I have done , the good or bad results and tweak from there. See when say UVB bulbs were bought , filters , parts , changes , type of food used , tank setups … Keeping some notes may help you , your call on that. Sorry for long post , I know you are trying at a young age and min cost , I believe in you , why I try to pass on some proven ideas and insight I used in my time. It's all worth it with turtles they are "Great pet's" especially my Piggley he's my pet , friend and child !