My wife had a longhorn and it was the size of a grape an it was actually braver and tougher than our tangs and would be right in the middle of the nori competing with the tangs. you have to be careful that when they are little that the current in your tank doesnt push it under a rock and get stuck or that it gets near a pump cuase the nori will get sucked to the bottom part of the pump or little wave maker and the longhorn will get sucked to it. My wife has had to save hers many times from these things. Ours grew to about 4 inches long. We have neon gobie cleaner fish that didnt harm her skin. But we have a lawnmower blenny that had a identiy crisis and took on that he was a cleaner fish and we caught him trying to clean the cowfish and in doing so, he caused her to have skin problems and ulcers and my wife medicated with melifix and it would look like it was doing better and then end up breaking out bad again. It never did recover and it finally died. Longhorn cowfish are not as toxic as some of the other cowfish. SOme cowfishes are very toxic. We would stress our cowfish many times cause it decided it like some of our corals and clams so we catch and put her in our 75 tank. When she got naughty in that tank, we gave her a time out in put her back in the 150 tank. So we moved her back and forth and each time the tangs in both tanks left her alone. We have a hippo tang, scopulas tang, powder blue tang, tentini tang, purple tang all in the 150 tank and they never harrassed her. In the 75 tank we have a atlantic tang and it never bothered our cowfish. After losing her, my wife had to replace her with another one. SO we bought another one the size of a grape and put her in an established tank with all these big tangs and they left her alone. What got our new one was the neon goby cleaners, they did affect her skin and we lost her the same way we did with the other cowfish. Skin problems. So we are waiting till the neon gobies pass away before we get another cowfish. SO with my experience with cowfishes, the tangs will be less of a threat to it than the cleaner fish. The cleaner fish will aggitate her skin and you may lose her that way. But as far as a longhorn polluting your tank and you losing stuff, you have a greater chance of losing stuff from putting in a coral and it polluting your tank an killing your fish and other corals. We have had that happen before and never having had 2 longhorn cowfish and we have had a hawiann cowfish and they have never polluted tanks that did any damage to themselves or anything else in the tank. But longhorn cowfish is my wife's favorite fish and they sure do have personality. Ours would stare at the door waiting for us to come home. Where ever you were in the house, it would be at that end of the tank watching you and wanting you to feed it. We fed ours by hand. As it got older, it would sometimes bite your fingers on accident when you fed it by hand and it was starting to take alittle piece of our skin off our thumbs. So would have hated to see what damage she could do to you when she got bigger. Everything we have read is that the longhorn could get up to 18 inches in the tank. when it gets bigger, it has huge pooh's. When ours was 2 inches long, it was poohing as much as our huge hippo. OUrs also got the taste for xenia. So becareful, it may have a taste for clams if you have any and also corals and ours did attack one fire shrimp after our file fish taught her. But she only did it to one of our many shrimps. The other shrimps she never bothered. So good luck and I hope this helped you.
It is about 4" long now! Gets along with My tangs just great !they dont fight with it no fish except My cleaner neons bug it It dosen't like the Neons cleaning it 
