Apple Snails Losing Their Colour?

[Spishkey]

i have to large golden apple snails, but in the past month or two they've gone from vibrant yellow/gold, to a very pale yellow/cream colour, almost white. not just there shell but their actual body too?

any idea why this is happening? are they lacking something in their diet? they eat mainly fresh veg and munch on turtle/shrimp pellets too?

 

[Littlemonkey]

How long have you had them, how big are they?

 

[saltynay]

What are the calcium levels in the water? Do the shells appear to be thinning?

 

[Spishkey]

ive had them about 5 or 6 months. they are rather large (bigger than golf balls if that helps?)
shells dont seem to be thinning, just lacking colour!

havent a clue what calcium levels are in the tank, should be good as the have cuttlefish bone in there (theres baby crays in the tank that need the cuttlefish bone too...)