One of my favorite "working fish" to keep in a turtle tank aren't really fish at all. They're crustaceans, specifically Palaemonetes paludosus, commonly known as the ghost shrimp or glass shrimp. These small, translucent scavengers are sold inexpensively in the aquarium sections of pet shops as feeders or tank cleaners, and they do a good job of it.
Ghost shrimp are basically little eating machines. That's pretty much all they do; and in the process of doing it, they also do a great job of cleaning the bottom of your turtle's tank of leftovers and other detritus. They also eat certain kinds of algae and diatoms. Best of all, they add almost no bioload to the tank. By preventing leftovers and other detritus from decaying, they actually improve the tank's water quality.
Another nice thing about ghost shrimp is that because they're usually sold as food, they're very inexpensive. I usually pay between USD $0.30 and $0.40 each for them if I buy them in person. You can also try breeding your own, but they are very challenging to breed. The nymphs are extremely sensitive to water quality and salinity and require special food. Unless you're a very experienced breeder of aquatic animals, you're better off just buying them.
Here are two videos of some ghost shrimp hard at work in one of my turtle habitats.
(Shot from underwater using the GoPro Hero 5 Black.)
(Shot from underwater using the GoPro Hero 5 Black.)
There are two problems with using ghost shrimp in a turtle tank.
One problem with ghost shrimp is that they may have parasites or bacteria. This usually isn't a problem if you buy your shrimp from a reputable shop. During the warmer months, I sometimes buy ghost shrimp from Amazon because it's a 50-mile trip each way to the nearest store near me that carries them (and that I trust to have healthy shrimp). During the cooler months, or if I have some other reason to make the drive, I buy them from Petsmart or Petco. I've never had problems with any of these suppliers' ghost shrimp.
The other problem with ghost shrimp is that turtles (especially younger ones) like to eat them. That's fine for the turtles, but not so much for the shrimp -- especially since you want them in the tank to help keep it clean. Luckily, there are some steps you can take to help delay the time when your ghost shrimp will become turtle food. You can't really prevent it altogether: Eventually, they're going to be eaten. But the following steps will give your ghost shrimp a fighting chance at a longer life:
Rocks that are concave on the bottom, real or artificial aquarium plants, or aquarium "hideout" decorations are all good ways to give your shrimp places to hide from the turtles and avoid being eaten. Just be careful to avoid creating traps in which your turtle can get caught and drown.
This is actually good advice for any turtle tank that also contains other creatures whom the turtles may be tempted to eat, such as fish or shrimp. Rather than feeding the turtles every other day as most keepers do, you can feed them every day (but feed them less food at each feeding, of course). The less hungry they are, the less likely they will be to eat their neighbors.
Whenever you replenish your supply of shrimp, feed the turtles immediately before introducing the shrimp. If they've just eaten, they'll be less likely to eat the shrimp that you just put in the tank (or at least they'll eat fewer of them). This will give the shrimp time to learn where the hiding places are.
All that being said, ghost shrimp are pretty good at sensing and avoiding danger. I've had them live as long as three months before being eaten, which is pretty amazing when you think about it. They're pretty quick, and turtles have to work pretty hard to catch and eat them; so as long as they're well-fed otherwise, most turtles won't work very hard to eat the shrimp. If the shrimp gets away from them the first few times they try to eat it (which the shrimp usually do), most turtles will give up and swim away.
Eventually, however, you should assume that all of your ghost shrimp will become turtle food. Even the smartest and fastest ghost shrimp will make a mistake one day, and on that day, a turtle most likely will eat them. If you're not okay with that, then you shouldn't use ghost shrimp in your tank.
What I usually do is stock the tank with roughly a third as many ghost shrimp as the tank's water capacity in gallons. So if I have a 75-gallon tank, I'll put between 25 and 30 ghost shrimp in it to start. They have very little bioload to speak of, and some of the slower ones will be eaten within the first few days, so this really isn't as many shrimp as it seems.
If the tank has more than one turtle, I add a few more ghost shrimp. The exact number really isn't critical. When I can't find more than four or five ghost shrimp in the tank, then I restock it with another 25 or 30. Typically I have to do this every two or three months.
Most of the time, you won't have to worry about feeding ghost shrimp who live in a turtle tank. Usually more than enough uneaten turtle food and "crumbs" will settle to the bottom of the tank to keep them happy. Eating the leftovers is a big part of their job, after all.
If you do have to feed them, there are commercial shrimp and invertebrate foods that will keep them well-fed. They'll also be happy with Reptomin, aquatic frog food or sinking fish food. They literally will eat almost anything, and I've never heard one complain about the cuisine.
You can use red cherry shrimp (Neocaridina davidi) instead of ghost shrimp if you like. They're prettier than ghost shrimp, especially if your tank has a dark-colored substrate. They're also a bit faster and slightly more adept at avoiding being eaten (and they won't hurt the turtles at all if they're are).
The biggest problem is that red cherry shrimp cost about ten to twenty times as much as ghost shrimp, and eventually they will become turtle food. It may take a bit longer, but eventually it's going to happen. The last time I checked, they were going for USD $3.49 each retail. You can usually buy red cherry shrimp on Amazon for a bit less, but it's still a lot of money to spend on animals that your turtles consider tasty food -- especially since you'll need quite a few shrimp if you want them to clean the bottom of the tank effectively.
If you choose to use red cherry shrimp in your turtle tank, I suggest you feed the turtles, and then remove them from the tank while you introduce the shrimp. Give them a fighting chance at finding the hiding spots before you put the turtles back in the tank. A few minutes should be enough time. You don't want your expensive shrimp being eaten as soon as you put them in the water.