Tortoises do need to drink water/soak themselves in shallow water on occasion, but they shouldn't live in it. Thankfully, you don't have to worry about that because you don't have a tortoise!
It looks like you have two turtles (not tortoises): The one to the right in your picture looks like a Reeve's turtle (
Mauremys reevesii), and the one on the left looks like a Florida red-bellied cooter (
Pseudemys nelsoni). They occupy completely different environments. Cooters like deep water, whereas Reeve's turtles like shallow water. The turtle that was staying on the basking area, if it was the Reeve's, may have been uncomfortable with the water depth in your tank. Which one did you think was a tortoise?
I doubt you want to read a long-winded post, so I recommend you Google each of the names above, and find some quality caresheets (i.e., Austin's Turtle Page, Chelonian Trust, etc.) Even turtlesource [not reputable] will give you some decent background information. I will, however, give you some basic ideas to start with (assuming I identified the turtles correctly):
1) Florida red-bellied cooters are largely herbivorous. You should begin to offer more greens to yours (i.e., red and green leaf lettuce). For more ideas, check out the "Nutrition" page on our main website. Avoid spinach and anything in the broccoli family (mustard, kale, cauliflower, etc.) Water and basking temperatures are pretty similar to RES. They also get huge, especially females, so you should consider getting a large stock tank (at least 150 gallons) for it.
2) Reeve's turtles prefer shallower water; around 6-8" with many easy-to-access haul-out areas. For an enclosure, your best bet is a 40 gallon breeder tank with the depth of water I just stated. Try to keep the water on the upper end of 75-80F, basking spot around 90F. They do well on a diet similar to RES--pellets balanced with aquatic plants and greens. Again, avoid spinach and broccoli-related vegetables.
3) Depending on how much water your enclosure holds, I recommend a good canister filter (Odyssea if you're on a budget, Fluval if you're not) rated for 2-3 times your volume. For instance, if you fill a 150 gallon stock tank with 125 gallons of water, you'll be looking for a canister that is rated for 250-375 gallon tanks. Please note that gallon "rating" like I just mentioned is different from "gallons per hour". Always go by the rating!
That being said, the Reeve's doesn't take too much water, and I've heard that many hobbyists keep them successfully with filters like the Tetra Whisper series (go no smaller than the 40i). People usually replace the carbon in the filter pouch with ceramic rings. Personally, if I had a Reeve's, I would use a 40i in conjunction with the 20i with 8 inches of water. This setup should do fine, especially with regular water changes (which we should all do, no matter what kind of filters we have). Again, this is for the Reeve's--not the cooter.
If anybody notices that I mis-identified or otherwise screwed up the above information, please correct me! You don't even have to do so politely.
Seriously, though, I haven't owned either species, but have researched them extensively in the past--as both were turtles I had considered purchasing at one point. So if current practices are different than before, please enlighten us!
Don't beat yourself up over this, agnes_silver--if you were a bad owner, then you wouldn't have bothered asking for help!
