Ok. You've got 2 Giant Gourami that are 1 1/2 ft long, plus 8 Tinfoil Barbs that are 4" long. 240 g 'sounds' like a lot but you've also got a couple of huge fish in there with huge bio-loads. And you're only changing 30% of the water once a week. I've got the same size tank with a 12" Oscar & 10 Clown Loaches and I change 70% of the water once a week. Nitrates get up to 7-8 ppm the day of the water change (I feed a lot). This is just an example for comparison but you've got 2-3 times the bio-load and you're doing less than 1/2 the amount of water change. You tested your tap and it had zero nitrates. Filtration has nothing to do with getting rid of nitrates, good filtration only turns ammonia & nitrites into nitrates. I bet if you start doing 50% water changes twice a week for a few wks your nitrates will start coming down fast. Then after that just do a 70-75% water change once a week. Really-it'll work. (Assuming your water is safe enough that you can do 70% weekly).
That's my take on it.
(Oops, it's '10' Clown Loaches in there, had to correct that).