The majority of reptiles that are kept as pets, can become ill only by bad feeding and poor hygiene. So, the turtles are reptiles and the same thing can happen to them too. Bad turtle feeding can result in serious diseases.
First of all you should know that water turtles are carnivorous animals, meaning that they mostly eat meat. Malnutrition apears when the turtle is fed primarily with vegetarian foods or inappropriate animal protein.
One of the most important thing to know is that turtle feeding must be done in the water. The water represents the vital part of the turtle's artificial environment. If you have a small pound in your turtle's home maid environment, then the water will easily become contaminated. You can feed your turtle in your pound but you should think about water quality, filtration and cleaning because it's a huge difference between the little amount of water hat you provide and the unlimited habitat that they have in the wild life.
The best thing you can do is too feed your water turtle in a place that is quite separated from their living environment. This is the best way to prevent the water contamination. This type of turtle feeding is mostly required for species of turtles that eat live food that they have to tear apart. When they tear the food there are little pieces of food that remain in the water and in short time they will tale to contamination. But there are also turtle species that swallow the food from one byte, like the snapping turtle or Mata Mata , that are allowed to feed in their living environment. This kind of turtles are considered "clean feeders".
Diet requirements vary from species to species and also they are influenced by age. The majority of water turtles are usually carnivores for all their life but there are species that when they grow old become mainly vegetarians. The best for you and your beloved pet is to find out exactly what kind of turtle you own and what dietary requirements should be included in the turtle feeding process. Next step after you are sure that you know what special feeding your turtle needs, is to find out what she likes to eat. So i personally suggest mixing different food items and observing the pet. You'll soon know what makes your pet happy. All in all don't forget that a turtle needs to eat varied food. So try not to feed only a type of food.
I will try, from my experience and different reliable documentation, to explain what is good and what's not for your turtle, regarding feeding.
Here is a general list of food items that I'm sure that are good for your pet: earthworms, nightcrawlers (make sure they are not raised on manure), mealworms, redworms, guppies, bait minnows, goldfish (occasionally), snails, butter lettuce (wash well) or melon and other fruit (find out what your turtle likes), banana, strawberries, peas, kibbles, Reptomin, Tender Vittels. Blueberries, dandelion flowers and leaves, vegetable scraps from your kitchen, tomato, cooked sweet potatoes...
You mustn't feed your turtle with raw chicken. It could be exposed to salmonella. try too feed it with cooked chicken. I have found sources that advise to feed your turtle with freezed raw chicken. That's completely wrong because the freezing process won't get rid of salmonella. If you want to give your turtle raw meat then try feeding her occasionally with squid, ocean fish or shrimp.
You can also include in your turtle feeding, commercial turtle food. But this foods are a little fatty and very rich in protein and i don't recommend daily feeding.
You could also feed your turtle with cat or dog food from cans, but i don't strongly recommend that. There are many good dry foods that you can use instead of any commercial cat or dog food. There are even veterinarians that would recommend this kind of food but i wouldn't give it to my turtle.
Also i think that it is very difficult for one to supply the exactly right mix of food for the pet turtle. So from time to time some vitamins are need. If they are well chosen they can't harm your pet. For more information on the vitamins suppliments that a water turtle should receive check out the Turtle vitamins section of this site. Please be confident on my recommendation because I'm just a turtle lover and you can be sure of anything that you read in this turtle care sheet