Which Salt do i use?

[bigfish]

At the moment im using Interpet Aqua Tonic Salt in my aquriums (have 3 so far) but its costing me a small fortune when im doing water changes as i have to add salt to keep it in balance because theres not much in a box, i was wondered if there was a cheaper version of salt but one that wont harm my fish as i know some salts contain a chemical that disagrees with the fish.

Could someone give advice of a cheaper alternative please.

Cheers Peeps

 

[saltydog]

BIG FISH, instead of water changing I have always used my (dioton filter) an just ad for evaporation.