Hi im looking after my friends marine fish and he told me to top the tank up if it goes low with the water he put in coke bottles, but now ive ran out and im just wondering is the water just dechlorinated tap water, or is it sumthing else? Thanks =]

no dont use tap water..you use RO water.either he has a RO unit plumbed in to the sink or he will get the RO water from his lfs for about £2 for 25ltrs

Ok thank you =] ill just leave it, it shud b ok for a couple more days. Im gettin paid to look after his fish...mayb i should open a buisness lol.

lol why not.what size is the tank the water could evapourate quite alot in a few days in a big tank..

I actually dont know, as they are in a hex tank, Its not one of those small ones tho its big, Id say about 80 - 100 litres, At his house they are kept in a 250 litre tank.

u probably will be ok for a few days then but dont leave it too long without toping up

I'd get some RO water and keep it topped up, if they're already in a smaller container then oxygen levels will be stressed and as the water evaporates the water salinity increases and that again reduces the oxygen carrying capacity of the water as well as introducing possible osmotic stress (and dehydration) on the fish.
If you're not checking salinity then at least keep the water topped up.

I assume this is a temporary home for them while your friend is on holiday?
Are there any invertebrates or corals in the tank?

ok ill have 2 go and get sum ro water 2moz. yep its just a temp for 2 more days. there r 2 hermit crabs, 2 clown fish, 2 goby and a algae blenny. do u think the lfs will let me buy enough water 2 fill 2x 2litre coke bottles as thts all i need. i have been testin salanity levels and it stayin at 31, it shud b 23, i tried txtin him but no anser,

1.031 sg ? Yes that's too high.

lfs should have a problem filling any container you give them. Make sure you specify "no salt, no minerals, just pure RO".

ok thank u very much = ]