turtles shells

Question
Actually, this is a question about turtles, but it should be a simple one for someone with experience!  I have had two small red-eared sliders for two years which were found in a local pond.  We live in NC.  Anyway, they appear healthy and well, just not growing much probably due to not being in an extremely large container.  My question has to do with their shells; even though I've started using purified water for their container (we use Brita filtering), they still get mineral deposits on their shells.  I've cleaned them before (carefully, of course), but was wondering if I could use a mild product on their shells to prevent these mineral deposits, namely a little Vaseline or baby oil.  I feed them a high-quality food which came from a reptile specialty store, but this store closed and I don't have them to turn to with my many questions. Thanks in advance.

Answer
Hello Patricia,

No, I would not use anything on their shells like the baby oil or vaseline & the reason is because it will wash off into their water supply & it wouldn't be good for them to ingest it through drinking.  
You can get some Novalsan solution & dilute it down to "wash" off their shells weekly.  It is non toxic & you can can take them out of their enclosure to bathe them so you don't get the Novalsan in their drinking water source.  
The only type of oil that could be helpful in keeping some of the mineral deposits off of the shell would be olive oil, or grapeseed oil, just an oil that could be digested if any drips off into their water.

I hope this helps out.

Tracie