Well it's been roughly 7 months since i've gotten my Cow Fish, Bessie. She's doing great, her horn is growing back slowly but surely. She doesnt seem to be ramming the hood any more. We got her when she was around 2 inches long including horns and today, 7 months later, she's nearly 6 inches long including horns. She eats like a cow and has a great personality.
I wouldnt suggest putting the cow fish in a 20 gallon. Once you account for the substrate, liverock, and anything else you may want in the tank, you all of a sudden have a considerably smaller tank than what you had first thought. My Cow is in a 72 gallon bow front with 1 3-stripe damsel and 1 yellow tank.
As far as "perfect water conditions", she is only 1 of 3 fish that has lived through everything. I lost many fish though the passed 9 months. Porcupine puffer, flame angel, snowflake eel, 2 peppermint shrimp, 1 cleaner shrimp, several snails, 1 figure 8 puffer, 1 domino damsel, and i'm sure there's more i'm not thinking of at the moment. Although I did have to treat her at least 5 times in the hospital tank for serious cases of ich. The last time I thought I was going to lose her. But obviously she recovered and is doing well. That was 2 months ago and hasnt had to have another treatment since. So now i'm leary about buying anymore fish in fear that it will have a negative effect on bessie and her health.
Jrittz