Sarah - not sure if you are still with us or not but just wanted to post a note about your tank.
Yes, your tank is a bit overstocked - don't always rely on what they tell you in your LFS because they are there primarily to make sales. It's always best to do your own research rather than believing everything they tell you. Some are better than others - but if you were told that you could put more fish into that tank then I'd be inclined to think this particular salesperson was not very knowledgeable on tropical fish.
Also, you mentioned you had a "loach" - what type of loach is it? If it is a clown loach then seriously this needs to be returned to the shop because 1) they grow to about a foot long (should never have been sold to you for a 30 litre!) and 2) they need to be with other clown loaches as they are sociable fish needing the company of their own kind.
For now, to keep your ammonia/nitrite levels down in your tank, you should be doing regular (daily or every other day) water changes (not the whole lot - just 25-30%). Don't wash out your filter sponge either - that can be done once a month (give it a sloosh in a little tank water when you do a water change - don't run it under the tap).
The cloudy water is a sign that your filter may not be properly cycled yet and there is an imbalance in the tank bacteria not enough to cope with the amount of fish waste being produced - a reason why a lot of new tank set-ups end up with sick or dead fish (known as "new tank syndrome").
Anyway, hope everything works out ok.
Athena
