There are many different species of aquatic turtles, and each of them has their own needs in terms of food, temperature, lighting, and habitat. This page is just an introduction that talks about the needs of aquatic turtles in general, and provides links to care sheets for specific turtle species.
All aquatic turtles share certain needs. The most important things that all aquatic turtles need are:
The exact feeding requirements for turtles vary by species. But there are several important things you need to know about feeding any aquatic turtle.
The first thing you need to understand is that aquatic turtles have to be in the water to eat because they need the water to wash down their food. Even if they take food from your hand, they'll still run or jump into the water to swallow it.
Many keepers recommend feeding turtles in a separate tank from the one they live in. This is mainly to prevent leftovers, which can foul the tank. I usually don't do this with dry, floating foods because it's easy enough to scoop up the leftovers with a net. Also, sometimes I have fish in the tank and they scoop up the leftovers. But when I feed my turtles messy food that's hard to scoop up with a net, I feed them in a separate tank.
The second thing you have to understand is that you can't feed your pet turtle dog food, luncheon meat, pizza, and so forth, and expect your turtle to be happy and healthy. Turtles have special dietary needs. Even though they're likely to eat almost anything that their keepers feed them, only a proper diet will keep them healthy.
The bulk of your turtle's diet should be food specifically made for turtles. Turtle food made by reputable companies like Mazuri, Zoo Med, Rep-Cal, Wardley's, and ReptoMin provide complete, balanced nutrition and should be a big part of your turtle's diet.
But no one -- not even a turtle -- wants to eat the same stuff all of the time, so it's okay to feed your turtle treats from time to time. You can also purchase several of the foods mentioned above and alternate them so your turtle doesn't get bored eating the same food all the time.
Depending on your turtle species and stage of development, they may like special treats like fish, brine shrimp, crunbled-up hard-boiled egg yolk, mysis shrimp, earthworms, or bloodworms; or plant foods like romaine lettuce, escarole, apples, bananas, and melons. Don't go tossing a whole melon in the tank, though! Offer your turtle tiny little bits of new foods to see if he or she likes them, and don't feed them more than the turtle will eat in a few minutes.
There are a lot of different opinions about this question. I believe that that juvenile turtles (up to one year of age) should be fed once a day, young adult turtles should be fed every other day, and older turtles should be fed every third day if there are edible plants in the tank for them to snak on in between. I usually feed my turtles as much food as would fit in their heads if their heads were empty with every feeding.
Others believe it's okay to feed turtles of any age every day, but that you have to feed them smaller portions, and still others believe they should be fed as much as they can eat in 15 or 20 minutes. Those ways of doing it are okay, too.
All keepers, however, agree that overfeeding your turtle is one of the worst things you can do.
In fact, one of the things you will have to learn as a turtle hobbyist is how to ignore your turtle's begging for food. This may sound cruel, but turtles are very intelligent animals who quickly learn to beg for food every time they see you walk past the tank. But overeating can cause all kinds of medical problems for turtles, so we have to learn to ignore their begging and feed them only as much food as is good for them..
You see, in nature turtles are "opportunistic feeders." That means that when they see food, they eat it -- whether they're hungry or not. This is because in the wild, turtles don't know when they'll get their next meal, so they pig out when there's food available just in case it's a long time before they get to eat again. But in the wild, they also burn off a lot of calories hunting for food. In a tank, they just get fat if they're overfed.
Pet turtles have their keepers to feed them, but they still act as if they were in the wild. So it's up to us to feed them only as much as they need.
A pet turtle living in a well-designed habitat, with proper heat and lighting, and whose diet consists largely of a high-quality turtle food, usually won't need vitamin supplements. Most turtle foods contain all the vitamins and minerals a turtle in a healthy habitat needs.
The most common exception to this rule is calcium, which turtles need in high amounts to build and maintain their skeletal systems and shells. Calcium can be added to a turtle's diet by adding supplements to their food; or it can be added to their environment by placing a cuttlebone or sepia bone in the tank.
Another exception to the rule is Vitamin A. It's actually a little unusual for turtles who are being fed a commercial turtle food to have Vitamin A deficiencies, but it does happen sometimes. Usually the first visible symptom is swollen, puffy eyes. A veterinarian can administer Vitamin A shots in the case of a deficiency, and there are also eye drops for turtles that you can administer to your turtles yourself.
Different turtles have different care requirements, so I'm before you get your turtle (or even if you have one already), you should read the Turtle Care Sheets page on the very excellent Austin's Turtle Page. Look for your species of turtle (or the species of turtle you're interested in) on the main page, and then select it from the drop-down list. You'll be taken to a page that contains specific information about your turtle's care, feeding, and habitat requirements.