You're not just guilty of overstocking. I'm not trying to be mean here, but you're the patron saint of overstocking.
Simply put, your tank is heading for a major and expensive crash. Your 9 seahorses by themselves are really too much for your tank. In addition to that, you have:
- A walking batfish (75 gallon minimum)
- A Fu Manchu lionfish (30 gallon minimum, but heavy bioload)
- A "few damsels here and there" (two or three would be too many, depending on what type they are)
- 2 small tangs (you should have 0 tangs in a tank that size - tangs need a lot of swimming room, and the least demanding need at least a 75g tank)
- 2 moonies (are these cape moonies/kitefish? If so, okay to have one in a tank your size... two is overkill)
- A combtail blenny (30 gallon minimum, so this one is fine)
- A lyretail anthias (Do best in schools of at least 3-4, and they need a lot of swimming space... 125 gallon minimum)
- A firefish (generally fine)
- A pygmy wrasse (probably fine)
- 3 (3!) leopard wrasses (You really shouldn't have any of these in a tank that's only 7 months old. They're very finicky eaters and require an extremely robust population of pods to survive. 3 in a tank your size would decimate your pod population and begin slowly starving to death)
- A foxface (Depending on the species, 70-125 gallon tank)
- A cowfish (Depending on the species, 90-250g minimum for one of these
- Not to mention, 9 seahorses
You have way too many fish in there, many of which require a tank much bigger than yours. The reason you can't get your nitrates down is because you have a massive nitrate factory in the form of 27+ fish, plus all the food that goes along with them. You can get away with that kind of a bioload in a larger tank, because you have an equally larger amount of live rock + filtration equipment to process those wastes. In a 55 gallon, though, there simply isn't enough room to grow bacterial cultures large enough to denitrify all the wastes. The bacteria that convert ammonia to nitrite, and nitrite to nitrate - those can grow pretty much anywhere in your tank, and they'll size up - to a point - to at least prevent the worst wastes from building up. The bacteria that process nitrate, however, are anaerobic - they only have so many places to grow (mostly in the deep crevices of your live rock). In a 55g, you don't have much space for them to grow, so with a bioload like yours, nitrates are just going to accumulate, and quickly.
Found an old thread on another forum that gave some rough numbers: An adult yellow tang would create approximatley 22 ppm of nitrate
per day. You have at least 7 fish in your tank that can produce the equivalent (cowfish, foxface, anthias, two tangs, lionfish, batfish), plus 20+ more that are going to be producing some significant fraction of that. So each day, your tank is producing probably 200-300 ppm of nitrate, if not more. And without enough space to grow sufficient denitrifying bacteria, the only place for that nitrate to go is your protein skimmer (probably not sized large enough to process that much waste) and your overall water.
Simply put, you need to reduce what you have in your tank, or else Mother Nature will reduce it for you. Rehome as many of your large fish as possible - the cowfish, the anthias, the foxface, the tangs, and the batfish all need a bigger tank. The seahorses really need their own dedicated tank where you can set things up to be more friendly to their needs. So, take out those fish and you're down to 12 fish, none of which are too big on their own for your tank. I'd try and fine a new home for the three leopard wrasses - your tank isn't mature enough to provide for them, and three is probably too many for your tank. Now you're down to 9. Probably rehome one or both of the moonies. Now you're at 7-9 (depending on how many damsels you have), which is much more reasonable for a tank that size.
For reference, I have the same size tank as you. I have 5 fish in it: a pink skunk clown, a ruby-headed fairy wrasse, a carpenter's flasher wrasse, a flametail blenny, and a chalk basslet. My nitrates hover between 3-10 ppm, and I feed pretty heavy. For optimal health in your reef, you want nitrates below about 20. Above that and sensitive organisms start having problems - the corals you've lost probably couldn't handle the nitrates. Fish tend to be more resilient, but even they will start to show signs of distress after extended exposure.
;Eggonface im dying over here lololol.
