Hi Ristof
It is important to remember that turtles are an animal that hide any illness until it is too late to do anything about it. Birds do the same thing. If a bird or a turtle were to sit around looking sick, they would be attacked by predators.
With birds, they tend to drop off their perch. With turtles, they suddenly become weak.
Illness in turtles can take a long time to show. Quite often, an injured turtle will die 18-24 months after incurring what looks like a relatively minor injury (unless it receives proper care at the time of the injury). During the 18-24 months, the turtle will look and behave normally, and show signs of repairing its injuries. But, it can actually be slowly dying. A post-mortem may reveal signs of septicemia, but that won't help a dead turtle.
So, whilst your turtles may appear to have nothing wrong with them, this is not necessarily the case. Turtles that are dying from kidney or liver failure do not show any outward signs. I've known people that smoke a packet of cigarettes per day tell me that they are perfectly healthy and that there is nothing wrong with them...
In my opinion, if you stick to a 'natural' turtles diet as closely as possible plus meet all the other needs of turtles, then your turtles should be healthy.
The occasional inappropriate food will probably do no noticeable harm. But, the bulk of their diet should be the foods that they would find naturally in the wild - these are the foods that they have evolved to eat. Pellets and frozen foods do not fall into this category.
If you'd like to email me some photos of your turtles, I'd be happy to give you my opinion on whether or not they look healthy to me.
Regards,
Michael.