Heyo! So sorry for the delay on my response.
I would reduce the time the light is on. I'd have to do a little more research on what light produces more growth and whatnot, because I don't remember the specifics. If you bought the lighting system with the tank, you're likely fine.
As the other person mentioned, definitely hold off on the water changes. Some bacteria growth is normal, but we don't want to disrupt the ecosystem too much. Tanks do have their ugly phase, but this seems a little to much to me.
Where did the shells come from? I personally had some that I got in Florida, and I ended up having to take them out. I had boiled them before, but I think that it contributed to the recent disruption I've had. If they're plastic, they're probably okay. Just make sure the paint doesn't flake - but that's not related to this stuff.
The one thing I would change IMMEDIATELY is the filtration type you are using. I have plenty of experience with filters and have done everything wrong in the book when it comes to them. TopFin is one of the worst brands I've personally dealt with. The circulation amount is way too low and that can definitely contribute. Truthfully, you'll have to open up the wallet quite a bit. In order to have good circulation and a well balanced ecosystem, you'll want to invest in a canister filter. I would 110% recommend the Fluval brand. The more circulation, the better. I personally have the FX4 for my salt. Spendy, but worth it.
Eventually get a better syphon but the filter is more of a priority than anything. Hopefully this helps!! Sorry you're having issues. Any more casualties?