harlequin tusk help

[Mimsypoopoo]

My beautiful harlequin tusk who is usually a very active swimmer is laying at the bottom of the tank like he's ready to die. I first noticed this like 4 days ago but the minute I walked over to the tank he sprung right up. But since than esp. in the last 48 hours he's gotten worse. He's still interested in food, and will get up and swim around when I'm watching him. This is not his normal behavior. The tusk has 4 other tank mates, Emperor angel, 3 stripe damsel, Hawk fish and a gold maroon clown. All these fish have been together for years. Its a 75 gallon reef ready tank, I run a eshops skimmer and have a good amount of live rock and normal sand. Please help, he's my favorite fish in the tank. I normally do a water change (8.8 gallons) every 2 weeks.

 

[54seaweed]

what are your water parameters,

 

[Sniperhank]

That is a pretty small tank for all those fish, especially with some of them being so large. I am expecting this to be stemming from parameter issues or aggression.