Hi val,
to the forum.
What you're experiencing in your new tank is a 'bacterial bloom'. It just means any bacteria spores floating around in the air are trying your tank out as a new home. Eventually it will clear up on it's own, or you can do some large water changes (although it often comes back if you do that!).
I wouldn't think about getting any fish just yet; here is why;
Fish produce ammonia, all the time; not just in their pee and poo, but also from their gills as they breathe. Ammonia is toxic to fish, and will kill them if it's allowed to build up in the water.
Luckily for us fishkeepers, there is a family of bacteria that just love to eat ammonia; they turn it into a substance called nitrite. Unfortunately, nitrite is also toxic to fish, but by a happy coincidence, there is another family of bacteria that eat the nitrite and turn it into nitrate. Nitrate is only toxic to fish at very high levels, and we keep the levels in our tanks low by doing a water change every week.
Until you have those good bacteria growing in your filter, which is what we call 'cycling', your tank is not safe for fish. The bacteria won't grow unless they have food, in the form of ammonia to feed on.
There are two ways of cycling a tank (well, technically the stuff inside your filter, but I'm sure you know what I mean
) fish-in and fishless.
In a fish-in cycle, you let the fish produce the ammonia. Obviously, you can't let the ammonia get to high, or it'll make your fish sick, or even kill them, so you have to test the water twice a day to make sure neither ammonia, nor nitrite, are building up and if they are, you have to do big water changes (sometimes two or three times a day) to keep the levels low. There is
always a risk of killing, or causing long term health problems, for your fish with a fish-in cycle.
In a fishless cycle, you use household cleaning ammonia, added to the tank, so you can grow those good bacteria before you add fish to the tank. We always recommend people do fishless cycles, as it is safer for your fish, when you get them, and a lot less hard work for you; you won't need to do daily water changes, as there will be nothing in the tank that could get hurt.
Shops would rather you did fish-in; for two reasons; the first is that a fishless cycle takes around four to six weeks, and most people can't be bothered having a tank with no fish in for that long. The second, more insidious reason, is that, if your fish get sick, they can sell you medications to 'cure' them (even though the problem is not with the fish but with the water) or, if your fish die, they can sell you more fish; either way, it's more money in their pocket.
If you click on the link in my sig, you'll find lots of useful articles on the nitrogen cycle (ammonia-nitrite-nitrate) and on cycling a new tank.
Hope that helps; do post again if anything's not clear to you
