So, is your NH3 zero or is the test indicating that there is some NH3 in the water? It should be zero.
I'm assuming you realize that any grouper and the sailfin tang are going to outgrow the tank, and depending on the species of lionfish, it might also outgrow the tank. Also, assuming we're talking about Grammistes sexlineaus (AKA sixlined soapfish), when stressed, this fish can exude a poison that can kill most or all of your tank. If your tank is spiking ammonia, it could be stressing the fish, which is dangerous in itself. Also be aware that this fish doesn't look nearly a pretty as an adult. As with many groupers, sixlines have an ornery tendency, and it just might be beating up on your lionfish since it's in direct competition for food. Speaking of food, the emerald crabs will be on the menu someday, as will the chromis.
Finally, you'll want to back way off on the krill and feed your fish more variety (table shrimp chunks, SW fish flesh, squid, scallops, clams, etc). Krill has a number of issues associated with it such as high fluoride and zero vitamin C content, both of which can affect the function and formation of the fish's mandibular structures (often resulting in lockjaw), as well as high thiaminase, which inhibits the fish's uptake of thiamine (vitamin B1). Lack of vitamin B1 results in central nervous system disorders such as anorexia (cessation of feeding), seizures, clamped fins, and an early death.
The first thing I'd do to help the fish is dose your tank with Vitamin C...since it's water soluble, you can simply dose the water (we use Brightwell's per label instructions for tank dosing). If your fish is eating, you can feed it Vitamin C orally via its food...for this, you can dose as much as the fish eats in a meal.
