Reddit mentions of OASIS #64226 Turtle Ramp - Large 16-Inch by 11-Inch by 4-1/2-Inch

Start getting more leafy greens into his diet. Romaine is an easy one (don't use iceberg). If he doesn't want to eat greens, then try putting a pinch of dust from the pellet container on them.

Good work with getting rid of the substrate. I'd also try filling the tank higher with water. Ideally you want twice the shell length in depth, but do what you can until you get a bigger tank (probably 65 gallons based on his current size). That rock that is currently his basking platform won't work with a higher water level, so you'll want something else. I've had good luck with this style - but there are a number of them that float and/or suction cup to the wall at an adjustable level. I left the rock you have in his tank (underwater) so he can scratch his shell on it and hide from things if he wants.

Also check the sidebar links to make sure you've got heat and UVB lamps.

Finally, save that 20 gallon tank when you upgrade - it'll work great for when you need to clean his tank.

I really like this one by Oasis. It's really sturdy and doesn't sink, plus my huge girl loves to dive off of the diving board. She's a RES and is probably 9 inches around. We have had to replace the suction cups, but it's worth it. I'm thinking about getting one for the other end of my tank as well.

https://www.amazon.com/gp/product/B0006L12HS/ref=ox_sc_sfl_title_11?ie=UTF8&psc=1&smid=ATVPDKIKX0DER

This is the best one I have gotten. They have smaller sizes if you don't need one this big. My Vinny is a growing girl. I got tired of buying one every couple of months when they would stop being able to hold her weight. I bought this one over a year ago. It's sturdy, very well made. I know it's a little pricy, but it's the last one you'll have to buy for a long time.

https://www.amazon.com/gp/aw/d/B0006L12HS/ref=mp_s_a_1_7?ie=UTF8&qid=1517461329&sr=8-7&keywords=turtle+basking+platform&dpPl=1&dpID=41vPk45-qeL&ref=plSrch