Or just get a brand new appropriately sized tank.... possibly a new filter....
and it looks like there should be a bigger land piece or larger land piece... Hard to tell if that one is actually big enough for both turtles to get up out of the water at the same time if they want to.
WWAAAAYYY bigger tank at least 75 gals a piece i have mine in a 75 gal and have had him for 5 years one side has flagstones staked up with a water fall and a large place for the convicts behind it and the other is flag stones staked up for basking mines about 2/3 full i had a craw dad climb out once and i found it in the kitchen under the stove!
I'd remove the gravel, especially for the moment. Turtles are inquisitive and curious creatures, and in such a small tank with little stimulation, may try eating it. Large pebbles (bigger than the turtle's head) are a much safer option.
That tank would be alot better if you would fill it up all the way. Build a above ground basking area. I have 3 adult Sliders in a 125 with a above ground basking area. You can also remove the gravel to help the water stay cleaner.