Here is a list of no nos:
Empty foods like lettuce and celery. They provide very little useful nutrition or fiber. They are fillers, a way to add variety but not nutrition.
Food high in oxalic acid like spinach, chard and rhubarb. Oxalic acid blocks the absorption of calcium. These foods cause the turtle to become calcium deficient even if a good source of calcium is provided. Example: You feed a calcium supplement on spinach. The oxalic acid in the spinach will prevent the turtle from absorbing the calcium. Even if you feed only now and then, you are still blocking the turtles ability to absorb calcium. Which is vital to turtle shells. The rhubarb the turtle ate a month ago, is blocking, months later.
High purine foods which include peas, beans and mushrooms should be used sparingly because they can contribute to gout if overused. Read sparingly as once every 3-6 months
Also avoid cabbage, brussel sprouts, broccoli, bok-choi, and kohlrabi (brassica family). In excessive amounts these plants contribute to kidney disease and goiter. Turnips, soybeans, radishes, rapeseed, and mustard also contain goitrogens. These should be used rarely. However, the addition of kelp to the diet may reverse the goitrogenic effect of the brassicas.
Raw meat will cause obesity in the turtle and it also does not provide the turtle with the nutrients it needs so much. Raw chicken can contain salmonella! Frozen fish inhibit the absorption of certain nutrients and might cause long-term problems. High levels of protein put an abnormal stress on the kidneys, and has been implicated in shell deformities such as pyramiding. Use meat as part of a varied diet, sparingly