I agree with Lindsey, too many large fish for that sized tank, especially with only one 50% water change per week.
On similar sized tanks with a smaller fish load, in the states I'd do 30-40% water changes every other day, or these in days Panama 25% everyday, to keep nitrate no higher than 5ppm. My latest nitrate test below

And filters don't break down nitrate (especially canisters), they produce nitrate by metabolism, the only reliable way to get rid of nitrate, is by doing water changes.
Plants, aquatic and/or terrestrial also "slightly" help reduce nitrate.

The tank above is the one the test was done on, it is stocked with about 15 fish, average 5".
I realize with certain fish, plants are impossible, so water changes are your best option, unless you do a planted sump, but even then, the combined weight of the plants needs to be much higher than the combined weight of the fish.
When in the states, I had a number of planted sumps like the one below.
