Temperate water supplementation?

Hello All,

I am in charge of the care of a handful of temperate marine aquariums.  These contain several crustaceans.  The water I use is natural sea water but the source is deficient in Calcium and Magnesium.  I have been supplementing calcium to boost the levels but I am wondering what, if any, supplements people use on their temperate tanks with crustaceans.  Does anyone supplement iodine as you would in a tropical reef tank?  If you do what dose do you use?  I am assuming it would be lower than the bottle says since the bio load of inverts is lower and physiology moves slower in general in colder water but I don't know if I should use a half dose, quarter dose, or none at all.  Are there any other supplement considerations I'm not thinking of?

Also, does anyone have any experience keeping Deep Sea Isopods?  I would be interested in any information people may have on their general care requirements.

NSW in NZ is down on calc and mag, i don't bother adding supplements in temperate tanks

Easier than dosing supplements would be just doing regular water changes with a quality brand of salt.  The trace elements in just a bit of new saltwater should be more than enough, especially if done regularly