hello keith, I spent 3 hours last night trying to answer you ,gave up about 3 am. I would get to the end press submit and nothing happened. Here we go again. If you are not going to get an r.o unit yet,go to your local fish store ,they will normally sell 5 gallon water containers. you can buy these empty, or they will fill them with r.o water for you at cost of around £2.50 a container full .the containers themselves cost about £6.00 each . but you can keep taking the empty ones back for refilling. ( bit of a nuisance that's why I said get your own r.o unit. )next to airate the water in these containers , you will need an airpump airstones airtubing (and non return valves to stop the water back tracking up the tubes and wrecking your pump ,If you turn your pump off for any reason ) assemble and and place the airstones in the containers ,turn on , you are now airating the water. airate your water for 24 hours .next you are correct in saying that you do not have to add any salt in the water you use to top up the tank to replace evaporated water, but remember when you are doing a water change you are removing the salt that is in the water as well as removing the water ,so this has to be replaced.what I do is pop a heater in each water container ,wait for the temperature to reach the same as your tank .(use a thermometer to check) add your salt slowly making sure you mix it well until you reach the same salanity as your tank, airate it for a further couple of hours prior to use ,check salanity again ,if its still the same as your tank ,then the water is ready to use.(turn off your heaters fifteen minutes before removing to give them time to cool down) While you are waiting for the heaters to cool down clean the algae off the glass and allow it to settle on the bottom of the tank,Get yourself a new bucket and use only for fishkeeping fiter your syphon at the bucket end with a fine net or tights etc and syphon the settled algae from your tank, once you have taken enough water out of your tank , at least 10 per cent take your rock out give it a good scrub and rinse in the water you have just taken out and replace back in your tank.top back up slowly, job done.on your system I personally would do a weekly water change. I would not add any more fish for at least a couple of months,I would install a protien skimmer And wait for things to settle down plenty of information about skimmers and r.o units on aquarist classifieds and e bay and any store that sells them,dont get disheartened almost everyone I know have problems when starting up and all the way through there is always something that can be improved upon . all the best grandad.