[Lorna]

I would not use sea water or salt from drying ponds as you can not guarantee the quality of the product you are getting. These may contain trace elements that you don't want to introduce to your tank such as heavy metals and minerals such as copper. I am not sure where you live that you are paying such a high cost for aquarium salt mix but a 50lb bucket of Tropic Marin sells for about $60 US at Dr. Foster & Smith. This would probably be about the equivilant of 20kg and make up about the same amount of salt water. Even so, it is part of the cost of the hobby and should be taken into consideration when venturing into this realm.