Hi there everyone I am new to this forum, but I need some help. I have a 55 gallon tank with about 50-75 pounds of live rock and 2 inches of crushed coral for the bed. As far as filtration I have two canisters filters one rated for 100 gallon the other for 40 gallon.The tank has been up for about 2 months as of now, when I started with the tank I helped it cycle with a clown fish from my established 24 gallon tank. After it cycled I added in 2 emerald crabs, 3 weeks after that I transferred in my blue hippo tang, mandarin goby, cleaner shrimp, two sexy shrimp, bubble tip anemone and a bunch of corals from the 24 gallon (The fish are all no more the 1 ½ inches long). After about a week and a half we bought a Spotted Unicorn Tang and two peppermint shrimp to help with our small Aiptasia Anemone issue. Since the tank was set up and finished it's cycle I make a habit of testing the water weekly to every two weeks. The water quality always tests perfect. Recently I noticed that I had an out of control Algae (green hair algae) outbreak. So I went to my local fish store to talk to them about a possible solution and they showed me a Sea Heare Slug/Snail that they said would help with my algae issue. So I paid the $8 bucks to rent him for a few days and bought a yellow tang as well. When I got home I acclimated them both to my tank and after the appropriate amount of time introduced them both. About 2 days later my wife came and told me that the Yellow tang we had just purchased had died. I didn't think much of it because the place that we purchased the tang from didn't always have the healthiest of fish. So we took the fish back and were able to trade it for a carpet anemone. We came home acclimated the anemone placed it in the tank and noticed that after a few hours that it wasn't looking very good. We decided to give it the night to allow it to get used to its surroundings but by the next day it had only gotten worse. We decided to take it back before it died in hopes to trade it for another fish or at least get our money back. As a side note we did get our water tested each time that we went in starting with the day we got the Yellow tang. By the time that we got the carpet anemone back it had died (it is a 15 minute drive from our house to the store.) The store was willing to give us a credit and we decided on a Blonde Naso Tang which we left at the store till the next day. While we were at the store I was speaking with the owner who was talking to me about the slug that I was renting, he then mentioned that it is a possibility when the Sea Heare dies it may emits a toxin that can kill things in my tank. So when I got home that night I checked on the slug and it was still alive so I decided to leave it in the tank and return it the next day. The next morning everything in the tank looked fine, the slug was up on a rock and all the fish were swimming around fine. My wife left to go and pick up the Naso Tang and when she got home (about an hour later) she noticed that our hippo tang was dead. She removed the hippo, couldn’t find the unicorn and noticed the mandarin was still swimming around so she acclimated the Naso and introduced him to the tank. About 2 minutes after introducing him to the tank she found the unicorn tang dead and watched as the mandarin passed as well. Quickly she removed the Naso before it died but noticed some weird behavior from the tang upon removing it. Such as the fact that it went from swimming curiously in the tank to hiding and then swimming toward the top of the tank in a drunken fashion. Once the Naso was out my wife started removing the dead fish and noticed that the slug had died too, yet there was no visible emitted toxin. In all the research that we did on the possibilities of what might happen if the sea heare died we came back with no real definitive answers. The weird thing is that all my fish died yet all the corals, invertebrates and even a bubble anemone we had was still alive. We got the water tested just to be sure and there was nothing wrong with it…So might anyone have an idea of why else all my fish might have died and everything else survived? Or would anyone have any info on the Sea Heare Slug/Snail? And if the slug is toxic how would I go about removing the toxin from the water? I did a 50% water change that same day but am worried to put my Naso in there after losing everything else. What should I do?? Sorry about the long story just thought I should be detailed. Thanks!!