Reddit mentions of API TURTLE SLUDGE DESTROYER Aquarium Cleaner and Sludge Remover Treatment 8-Ounce Bottle

I'm under-qualified to properly answer your question, but since no one else responded, I figure I'll share my thoughts.

I've heard that when getting a new filter, sometimes it'll cause an "algae bloom". This is my biggest guess as to what it causing your issue. Unfortunately I'm not sure exactly what you're supposed to do, other than keep doing water changes, and be patient until the water has adapted to the new filter.

I also have a SunSun, though its the 303b model. From my research, the media that comes with the filter is fine, but you can actually buy more and fill it up if you have extra room in there. I think what it comes with wasn't really enough to fill the entire filter. You can try buying some extra media and putting it in, maybe that would help.

I always get a calcium build up on my water line. I try to wipe it off once a week with a scrubber (no soap! just water). Should be harmless. Last recommendation I can give is to buy some water cleaner, like this one or something similar. It's supposed to help reduce odor and get the water cleaner. Definitely worth a shot.

I've been in the turtle game for about a year now, recently I've upgraded my 55 gallon canister filter to 150 gallon one called Sunsun HW-304B and wow has it been an improvement. Turtles need 3x the filter so if you have 55 gallon tank you're going to need one rated for 150-200 gallons which canister filters are preferred since they can last a long time without cleaning and provide better suction.

If you do go the canister filter route I would get this media kit that has everything you need to started.

As far as chemicals go I would stick with TetraFauna Aquasafe and API Sludge Destroyer, use only a little bit since that will make your water cloudy if you use too much.

To pick up food waste and whatever else get this long pole fishnet instead of those short ones that they sell at pet stores so you can get in there really deep without shoving your arm in.

Turtles also need 2 types of light UVB and heat, I found that this combo lamp works fine.