I'd be hard pressed to believe it. I wonder where they got their information from. Acidic food draining calcium from the fish just sounds rediculous, IMO. Especially since they didn't even explain 'how' it happens.
I'd look for more reliable, scientific sources for info.
Spinach is one of the best foods you can feed your herbivorous/omnivorous fishes. It is full of vitamins A, B1, B2, B3, C and K.
This is not an attack on you, BTW....just your source of info.
If anyone else has anything to share on this subject, then please share your knowledge.