Apple snail shells

Hi everybody,

following the thread about apple snails, I wonder if it is possible in my setup 
to add one or two or if my water is too soft for them to develop healthy shells.
My setup: KH/GH 3-4, PH (via CO2 injection) 6.7-6.8, tank size 60 gallons,
temperature 80-83 F, fish are Discus, neon tetras, corys, SAEs, ancistrus.

I do weekly 50% water changes with 95% RO and 5% tap water and add Dupla 
KH/GH Bildner and Duplaplant (the 5% tap water have been recommended to me
by Dupla by email - they seem to respond to questions written in German). 

I have to use mostly RO water because of the high nitrate content of the 
tap water to prevent algae. I read that some list members add lime to their
water for the snails. I am not really skilled in water chemistry. Does this
increase KH/GH? I prefer to run my tank at low KH/GH levels so I don't need as
much CO2 and my fish and plants seem to do better so increasing hardness is not
really an option.

Greetings and thanks, Andi from Munich, Germany (about 0 Celsius and ice)