I have had 5 roseline sharks in my 75 tank for the last 2 years or so. I change 75% of the water weekly sometimes, but usually it is only 50%. When I first got them it was a fish only tank, and now I am running it as a reduced EI tank and I have done it with and without CO2, with and without aeration, with a canister filter, and now with aquaclear HOB filters. They did fine in all combinations. I have always had brilliant coloration from the roseline sharks, and their behavior seems very consistent and healthy during and after any water changes, fert dosing, adjustments in kH or gH (I have never increased it by more than 2-3 degrees hardness at any one time), etc. My tap water is very soft with a kH of 1-2 and a gH of about 2-3 seasonally out of the tap. The temp of the tank fluctuates with the house as there is no heater in the tank. It is usually 72 in the morning and 74 in the evening as a result of the light's heat output during the day. During the summer the tank can get up to 78-80 since the house doesn't have A/C, but I have never had it any higher. I feed the fish mysis shrimp and bloodworms mainly, and all of the fish are very active and if they lose a scale, tear a fin, or get hurt somehow they heal within a few days. The only problem I have had is that occasionally the roseline sharks pick on each other a little, but never on the other fish in the tank. It usually only lasts a few days and I am not sure why it happens exactly. It might have something to do with mating behavior, but I don't know as I have not tried to breed them to know much about their behavior when breeding.
I have heard that roseline sharks are actually a member of the Koi family, and that's why they have the slight whisker on each side of their mouth like Koi do. I have not verified that though. I have a large Koi pond outside that also has goldfish in it, and I can tell you that the Koi are much more sensitive to water quality, oxygen levels, etc. than the goldfish are by far, but it is by no means hard to keep them all happy and flourishing. I hope it helps.
-Jeremy
P.S.- I figured that I would add that the roseline sharks were about 1.5" long when I bought them, and they are between 4-5" long now. They might be more sensitive initially if you buy them when they are smaller. I have seen them at the LFS as small as 3/4" long or so.
