Water has to be heated and u can buy sea salt just add to tap water and u have salt water need a week or 2 for the tank to work its self out and test water before you can put fish or eel in.
TAP WATER IS NOT RECOMMENDED FOR MARINE TANKS DUE TO CHLORINE, CHLORAMINE, PHOSPHATES AND ALOT OF OTHER NASTIES IN THE WATER.
BUYING SALT IS FINE IF YOU PREFER TO GO THAT WAY BUT YOU WILL NEED RO WATER TO MIX IT UP WITH.
With a 20kg bucket of salt costing more than $100 it's an expensive way to go. you will also find nsw has the correct levels of kh, calcium etc and if you are doing regular water changes you won't ever need to use any supplements with nsw. Unless you have a full reef tank which then calcium such as kalkwasser would definitely be needed.
Sorry, didn't realise caps was on up there.
A week or 2 is nowhere near enough for a marine tank to cycle. It would be a minimum of 4 weeks - adding any livestock before then is plain cruel.
As for using natural sea water, the die off from plankton has minimal, if no affect at all. I have changed 300 litres out of 500 and have had no die off - if there were die off ammonia would be present.
Most people filter their sea water as they are pumping it into containers and another alternative is to buy it from the aquarium store already filtered for about $5 per 25 litres.
Marcus please don't go giving people advice on things you aren't quite sure of yourself.
Setting up a martine tank isn't as easy as filling it with tap water, chucking in some salt and 2 weeks alter adding livestock - there is alot more to it than that and if you have been told otherwise I would suggest you do a bit of research as I would hate someone to be taking your advice right now.
Remember mother nature is very clever and replicating what she has done is not exactly easy.