Tank water
 

I went to my lfs today to see if they sold ro/di water, on there sign it said De-ionized water, 60 cents per gallon, is this considered reverse osmosis? Also they sell water pre-mixed with instant ocean sea salt as well as water pre-mixed with reef crystals, which water should i buy?

20 gallon tank
2 ocellarus clownfish
Mushroom coral, more specifically Green Ricordea
5-6 wpg

I dont know if i should mix my own salt, use reef salt or instant ocean, or buy a premixed saltwater

Depends are you going to dose your tank with all the chemicals it needs. If so go with the INstant ocean.

should i buy ready mixed or should i mix it myself, also the sign says de-ionized water but doesnt say anything about R/O. Is that normal? Is .60 cents a gallon really worth it or can i find a better deal elseware

I put DI water in my tank only. I also pay .65 cents per gallon.

I make my own RO/DI water. RO and DI are very different creatures. RO water removes particles much as a filter would. This tends to not mess with the PH, alk etc of the water. Di on the other hand strips the water of every last bit. Have your shop use a TDS meter in front of you to see that it reads no more then 002ppm or don't bother spending money on it. The resin needs to be monitered closely as it works very well. To sum up the answer, DI is more pure then RO and will work well in your tank.