If it is calcium deposits, try vinegar and it will help to remove them with work and I mean work and it will not make your aquarium unusable. It will also disinfect the aquarium to use vinegar in it and then wash, wash, wash with clear water. It will take a while to take the deposits out. There is a product to remove calcium from an aquarium that is aquarium safe but it is also a bit expensive and does not do a better job than the vinegar so that is why I do not recommend it.
Rose