There could be many reasons why the water is cloudy
For starters water changes, you are doing far too much too often, doing 90% water change means your tank has to cycle all over again.
As the tank is new it could be the substrate, how high is your PH?
If you find that it is high then it very well could be the substrate...that’s as easy as washing it out.
The second reason could be Bacterial bloom, this would come about due to the large water changes for your size tank. In a larger tank you could do 30 percent water change and have a small spike in water balance but because you are doing it in a large body of water then it has minimal effect. But in a smaller tank then the effect is much more severe. And every time you do large water changes you get a bacterial bloom and it takes a while for the tank to establish the needed bacteria to clear the bloom. Since this is easy to test and all you have to do is do smaller water changes and wait.
Lastly and one that has been mentioned already is fish stock, having a 30ltr tank restricts how much stock you can put in your tank.
Easiest way to find out is by using the aqua adviser and put in your tank dimensions and filter as well as your fish and it will tell you what your maximum capacity is. (here is the link for you)
I would test your tap water on its own and leave your tank for a week, do a 10ltr water change. and then after a week re test the water again. also keep in mind anything you add to the water to help would be super potent due to the size of the tank.
What substrate are you using? and what type of food are using?
the filter may not be the best but it's easy enough for that tank, bad filters don't filter well and when pushed too hard or are over loaded turn your tank to hell.
But in your case the filter may not be great but it's more than enough for your tank.
I have an Eheim pro external canister filter that is not a super jet but it cleans the water like nothing else and that’s where the quality is.
If you have a choice in filter again I would suggest a canister filter as you only have 30ltr tank and anything in the tank is taking the room away from the fish. I also think canisters a quieter as well. you can pick one up for around $80-$120 bucks that would be more then suitable for your tank.
Hope this helps.
The price is about right, I have seen them cheaper, it's suited for a 45ltr tank. If you can always go higher. I have a 175ltr tank and my eheim is more suited for a 350ltr tank.
if you can afford it then def go higher. you can never have to much filtration. here is another tha would be suitable for your tank.
http://thereefshop.com.au/index.php?main_page=product_info&cPath=10&products_id=1152 I purchased my Ehiem through these guys and they were fast and reliable.