Jenncarrol...I used to live in New Orleans before moving out west in search of gold
I beleive you have several issues going on and I will give you my opinion. Please know it has been awahile since I sucked the crawfish heads so my brain ain't what it used to be..
You are adding too much too fast. You should wait about a month between additions. Also, do not overcrowd the tank as this is prime for ich to spread assuming it is in your system. Tangs are big poopers so they will but a strain on your biological filter in the short term which creates a condition for disease due to stress from descresed water quality.....provided disease is present in your system
Unfortunately, taking home the Blue Hippo before you witnessed it eat is a crap shoot. This animal may have been just removed from the ocean and has yet to adapt to living in a cube. Also, larger to medium size tangs need 6 feet of swimming space in order to thrive for a long time and be happy. As he gets bigger he may not be happy in your tank.
Too late now but placing copper in a system is a big no-no as it is difficult to totally remove. You may have problems keeping inverts alive in the future.
Live rock is one of the best things you can add to your system. It provides a natrual food source for some animals plus increases your biological filtration as it provides the perfect enviornment for beneficial bacteria to grow. I beleive it provides a more natrual home for you fish. ALways be sure the live rock is cured. If uncertain do not add it to your tank without recuring,. Uncured rock can cause another cycle and you could lose all your animals.
Concentrate on keeping your water quality constant, temp at about 80-82, plenty water movement, a good nutritional diet, feeding garlic soaked food a few times a week and vitamins.
You want to keep the immune system of your fish in good shape as they can kick mild cases of ich.
UV steriliers will only work to kill the free swimming stages of ich. It does nothing for the parasites living on the host fish. SOme feel it does little but give the parasites a sunburn...I beleive it can help in keeping ich under control....but it does not cure ich.
It takes about 45 days for ich to cycle. I have no idea if the copper you used totally wiped it out. Early March to now indicates the cycle could still be "active". Therefore, it is possible that the disease is still present which is why it surfaced so quickly. Also, please test your water for copper as it is tough on tangs.
Sorry I rambled.....GEAUX SAINTS!!!!!!!!
