an old and accurate post from a member on pufferfish....
Despite what I have read here, and form other people who have never owned Cowfish, they can be kept alive long term if taken care of properly. I know because I have had 2 that lived longer then 7 years.
The secret is to treat them as an almost species only. You cannot just toss them in any old tank and expect them to survive. They get along with other slow moving fish but the tank should be geared for Cowfish, defiantly not as a reef.
What dose that mean? Well for starters cowfish have a special ability to hover in the water column. They do not like high flow jets of water or the will not be able to hunt for food. However, they eat and poop a lot. I mean A LOT! Lots of water changes and cleaning. Basically, they need a high volume of water turnover though the filters, but water movement in the tank needs to be low. Get the biggest protein skimmer you can afford and be prepared for small water changes once a week.
They need to eat every day, twice or 3 times if possible. The diet mine did so well on was primarily frozen Mysis. That was supplemented with other shell fish like clams and occasionally some algae. Also of course some fish vitamins like Vibrance, or Selco on the mysis regularly. If you skip a day, don't be surprised if your shrimp are missing. The turn into sharks and start sampling everything from snails to corals, which can be deadly to them.
They do ok with other slow moving fish such as a Yellow Tang or Blue Chromis are ok but not anything faster or bigger. Clean up crew is recommended and mine actually let cleaner shrimp clean him. NO STINGING CORALS, they do not have scales to protect them from even the most minor of injuries. While they do have a hard bony exoskeleton, its covered with skin, which like people, is considered an organ.
They are also prone to ICK and while this is a common problem in fish tanks, you cannot use regular methods to cure them. Cowfish do not have scales, they have skin. This means most ICH meds that contain Copper cannot be used without killing them. Hospital tanks are also tricky because moving cows to another tank is very stressful. They are a very hardy fish once they have settled in a display, but the highest rate of death seem to come form assimilation, extreme water changes or moving them to another tank.
They grow up to 18" and can live up to 20 years in the wild! So when you read they need a big tank they mean it. That cute little sugar cube size fish will be 6 inches in just a year. You should plan on a tank at least 100 gallons or more after that or the bio-load will become unmanageable.
As for poisoning the tank, I have not found this to be true from talking to actual owners. I only hear this mostly from internet rumor or friends of friends. Don't get me wrong, I am sure they can do this, but I suspect it only happens under more extreme stress such as being attacked by a larger aggressive fish. When my first cow died, I removed it about 1/2 hour after noticing. It did not nuke the tank or kill off any other fish. If their tank is peaceful, this should never happen.
Finally, these fish are very smart and need attention. They will watch you move around the room, spit jets of water out of the top at you and fallow your finger around the tank. They will make noise to get your attention by splashing water or fan out their tail if you respond. They care more about what is going on outside the tank then inside. Smart creatures generally need more attention and interaction to remain healthy.
On a side note, they can hear sounds, through an inner ear, smell food in the water, taste through their skin and unlike most fish have the sense of touch on outside of their body like people do with their skin.
Warning, if you do decide to get one, you will fall in love with them like you do with your favorite cat or dog. You will be hart broken with then go, but they are so people friendly, they will make you smile everyday. Take good care of them they are suppose to live a long time.