How often should I test the water in my tank? We change about 30% weekly (400L).

Generally, you should be testing your tank monthly. If it's a newer aquarium then you want to test in more often, weekly perhaps. The same goes for when you get a new filter or add some new fish. Whenever you are making changes you want to monitor the nitrogen levels to see how well you little environment is responding. But routine maintenance on an established aquarium would call for testing your water monthly. At least, that's how I do it.

Yes, can't really add any more to the above. If you see any fish acting strangely, do a water test. The way I like to explain testing the water is like going to the doctors, he will test your blood pressure, maybe take some blood. This is very much like testing the tank water, if there's something wrong with the water, this could affect the fish.

what is the correct water readings for the oscars. got quiet high reading on some which worry me and i need to srot it out asap
cheers in advance drew

Tell us what your water readings are.

amonia - 0.5 (just changed 45% of the water
high range ph - 7.4
ph - 6.5
no3 - 20
no2 - 1.5

Okay, nitrate levels are okay, pH is okay. Ammonia is not okay and neither is nitrite. Nitrite is actually very high, that needs to be dealt with. Looking at your tank makes me think it's not cycled properly. How long has it been set up and how many fish have you got in it?

just took all the old fish out not long ago. i used to have a plec and 2 angel fish along with sum tetras.

got rid of them all and put the angel in a seperate tank by themselfs as they are breeding pair and agreesive all the time.
its been running as a comunal tank for about 7 month now. done a 50% water change 2 days befor the oscars went in also for the two previous days there was no fish at all in.

could you please tell me how to sort these levels out

First of all, we need to work out why your levels are like this. A tank that has been established for several months should be fully cycled with no ammonia nitrate at all. When you clean your tank, how do you do it? Have you cleaned the filters at all? If so, you don't wash the media out under tap water do you?

How big is this tank? How many fish were in the tank before you put the Oscars in and how big are the Oscars? What I am suspecting is the amount of waste that the Oscars produce is far more than the fish than the previous occupants of the tank. For this reason, the bacteria are having a hard time dealing with all the ammonia and this is why you are getting these readings. If this is the case, you've got to let the biological filter catch up with the amount of waste that is being produced. I would suggest that you do 25% more to change every other day until the biological filter is able to cope with all the waste being produced. However, this is just a hunch, may not be right.

i did used to clean the filter with tap water till i learned on this very forum that you should use the tank water.
the tank is about 4 ft by 2 ft by 2ft deep. the oscars are between 8 inch and 7 inch from nose to base of tail.

erm around 9 fish befor
medium pleck
2 medium angels
and some tetras tiny things
as for the cleaning i normaly clean the glass first with one of them magnectic pad thingys then clean gravel with a syphon vac and also take a small amount of the water out extra which i now use to clean the filter. im thinking on putting the other spare filter in which is one of those underwater ones which produce no bubble just clean.

Putting another filter in your tank will not help your situation at the moment, that will take a few weeks to cycle and start doing the job. I'm pretty confident in saying that because you've added the Oscars that produce more waste than your previous fish, this is why you are getting these higher readings. Hopefully, it shouldn't take too long for the biological filter to catch up. Just keep up the water changes either every day, or every other day. I would advise you to go and get some prime if your shop sells it, this will detoxify ammonia and nitrite and make the environment and little safer for your fish.

its a fairly good shop and he sell everything so i should think he has some. so i ask for prime yeah

and i realy apreciate your help mate i will keepyou updated on the levels as ill check them on suda too befor i feed