That looks like a 19l Pets at Home tank? To be honest that's way too small for a goldfish, they produce a lot of waste! Also if it's only been set up a couple of days it's also probably not cycled, meaning there are not yet any bacteria in the filter to break down the fish's waste (saying that, it's probably still better than the bowl!)
If you know anyone with a tank local to you, best thing to do would be get some of their filter media and stick it in your filter - that way you're kick starting the bacteria in your tank.
Failing that, frequent water changes are your best idea to try and keep the water as clean as possible!
If he's not happy with the flow from the filter, you can stick a bit of filter foam over the outlet - held on with an elastic band or cable tie. We had to do that when we had our Betta in one of those tanks.
The tank also looks quite bare, so a few more places to hide would do him some good too - either more plants (fake or real - real will help with the water quality if you have adequate lighting) or some ornaments/caves etc.
Most important thing would be to focus on the water quality at the moment however, possibly grab some test strips (or a liquid kit depending on how serious you are about the hobby) and see what the readings are.
Edit:
Update on our tank as well.
Looking good so far, the new wavemaker is in the opposite corner to the existing one - rear left about half way up. Also repositioned the return outlet so it's flowing against the back of the tank just below the water line.
Got a new skimmer on the way and a TMC v2 auto top-up fitted - so far used about 8L of RODI topping up, so not too bad.
Wondering if we've had a quick cycle or it's not started properly yet. Put a king prawn in (after using the rest for paella
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) to generate some waste, and been ghost feeding the tank since the weekend - it's been less than 2 weeks, but ammonia is back down to 0ppm, after peaking at 0.5 (it may have been higher last weekend, but we were away for a few days so didn't test it), and nitrite is just above 0ppm (
very slight hint of purple to it), after hovering at 0.5 for a few days. Nitrate was between 20-40ppm (my GF and I couldn't agree which colour it was
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), but just done a 10% water change to bring that down a bit.
All testing done with an API saltwater master test kit.
Not had any diatoms yet - going to wait another week at least while testing to make sure it's stable, and then maybe add a couple of snails.
Pretty sure those wavy things are feather duster worms - spotted another couple on top of the rock, along with what appears to be one gradually building up it's tube
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