What the 'others say' are correct, Bio balls are a nitrate factory. Remove them slowly from your tank (not all at once) Remove a little every three days or so..
Is it possible to post a pic of your sump?
My quick noob reefkeepers pennies worth - Been in the hobby for more than 10 years mainly FOWLR. Bio balls / Ceramic Rings were all over my sump. Took the gigantic leap to reef about 2 years ago. Nitrates became public enemy number one.
Based on tons of reading and learnings from across this forum and its guru's ive switched to a DSB and started removing my bio-balls over the last few days. I wait for the day when my nitrate test kit changes colour (which in my case is from red to yellow) then I will open the champaign.
I guess the proof is in the pudding - so I will wait for the emperical evidence. That said all evidence points to NO:nono: NO:nono: NO:nono: for bio balls.
Bio-balls come from the "old age theories" of keeping fresh water fish. It was indeed use in the times when reefkeepers did not keep as many corals, as in today's times. And also not as many SENSITIVE corals.
The draw back of ceramic noodles AND bio-balls, is that yes - it works to break down ammonia into nitrites and nittritest into nitrates - BUT, it is NOT conducive of breaking down nitrates into nitrogen and oxygen and hydrogen... this is where the DSB works best...... it goes all the way in the WHOLE nutrient cycle.....
In fresh water tanks, aquarists used to have many freshwater plants - which took up nitrates and phosphates.
Therefor the implementation of "algae scrubbers" in marine tanks - to take the task of fresh water plants - to "use up/export" nitrates/phosphates from the water.....
Reef Invertebrates from Anthony Calfo and Robert Fenner doesn't even mention bio-balls if I'm not mistaken. They only speak about good quality live rock, live sand and properly maintained DSB's and nutrient export via macro algae. Obviously skimmers come into play as well.
My advice - NO BIO-BALLS OR CERAMIC RINGS/NOODLES in a marine system. Period
I'll also go with a decent DSB. Only downside is that it takes 6 months and longer to mature to show actual results. Until then don't overfeed your fish and do regular water changes together with blowing detritis off the live rock say on a weekly basis. You can also stirr your substrate now and then. Advantages is that uneaten food and poo poo don't get a chance to settle creating water quality problems and the corals love it
You said you got a FOWLR system.
No special corals. So with only fish you can get away with bio-balls. Just like the LFS does it. (That is why they still sell stupid sumps with bio-balls - because it works for them...)
Your Phoshate and Nitrate levels will always be high.
So your choise.
But one day you want some softies, the sps then.... For that you need a DSB. And a DSB can take up to 6 months to become fully active. So, my advice, start now planning a sump change or replacement.