Hi, our Oscar has been behaving oddly for 2 weeks now, he is not eating, and is spending alot of time hiding, tucked in behind a decoration in the tank, he also appears to be breathing heavily, (not constantly tho), we can not see any discolouration near his gills. he also has these mad swimming in circle fits. I have noticed when the aquarium lights go off, he comes out of hiding and swims more in the tank.
He has a mark on his head which has semi healed. photos attached

current water readings:
ammonia 0
nitrite 0
nitrate - 40-80 - have done 2 water changes in the last 7 days, once 30 litres and last one 49litres 3 days ago. We were away for 3 days this weekend, and the fish were not fed. I cant quite make out the reading, it looks closer to 40, but I'm not 100% sure.
I checked the tap water nitrate & it is 0!
PH 7.0
tank temperature 27
tank is Aquaone AR980, which says its 240litres, it has its own filtration system.
he has 3 silver dollar tank mates
We first noticed oscar was upset after we did a big tank clean a couple of weeks ago. we are very worried about him and dont want to lose him. He is about 2 years old.

Where do I start, your way, way overstocked. your tanks about 55g and is only large enough for a single lone Oscar. most likely your filter is going to be rated for your tank size (55g)and that means the filter will not cope with the bio load your fish are putting on it. you need a much larger tank or to re home some of the fish.

most likely the problem is going to be ammonia/nitrite spikes and high levels of nitrate. short term your only hope is going to be large water changes of 25% every few days.

In the short term aside from the frequent water changes I would invest in another filter--a canister filter if you can afford it (about 405 GPH) or at the very least a 400 GPH hang on back to assist the filter you have. Your filter is rated for 55 gallons so we're looking at an estimated actual performance of around 35 gallons. Too small even for just the SD's--much less an oscar.

I agree with marcus and johnny. you have too many fish for your tank. So look into rehoming the SD's.

Also the filters that come with tanks are of the right size for keeping community fish in the tank. Oscars are messy and require much more filtration. You need a filter that is rated for a tank twice as big as the tank. So in your case you need a filter rated for 500litres. I would go for a fluval external filter. Get this ASAP

You oscars problems are as marcus said probably due to poor water quality, probably some ammonia exposure and high nitrates. While waiting to get the filter do daily 25% water changes. The mark on his head will heal quickly with good water quality!

I dont understand, why am I getting 0 readings for ammonia and nitrite constantly? I check it every week- why don't I see high readings for these.

Then the problem is most likely the prolongued exposure to high nitrates.

they should be kept low as possible, and your tend to be hogh most of the time.
areading of 40 is allready starting to be toxic for your o and will have it's effect on his health.
Tanksize, filter,waterchanges and bioload combined should keep nitrates preferably round 20...and deffo not spiking to 80!

that helps, thanks I didn't mean that the nitrate was 80 - I cant read the colour from my chart well - it looks like 40 but it also looks like 80 if that makes sense? the colours are so close on the chart its hard to tell what it is. Maybe I should try another brand of test?
I will do some more water changes and we will look at a new filter as soon as possible. thankyou