Welcome to Tonmo. I am a scientist in the US who has worked with captive nautiluses in a research facility for about four years. Generally we discourage people from keeping nautiluses as pets because they are difficult to keep healthy in captivity, and because we don't know enough about the health of their wild populations to be confident that a pet trade is supportable.
A chiller is essential - even though they are from the tropical ocean, they live in deep water that remains cold. A temperature of about 15-18 degrees celcius is necessary, anything above about 23 degrees will be fatal. So if you are keeping it in an outside tank you must have a chiller. The size of the chiller you need will depend on the size of the tank. For one nautilus, no smaller than 150 gallons, and a vertical tank is better than horizontal. If you want to keep more than one, you should expect to have a tank of about 200 gallons. People keep them in smaller, but since they routinely swim hundreds of meters up and down every day, bigger is better.
It sounds from your posts that you have not had a saltwater tank before - (apologies if I have read that wrongly). I definitely don't recommend nautilus for a first-time keeper, they are very sensitive to water quality and can become ill without showing many effects. An experienced keeper would be more likely to know if there is something subtle that is wrong.
You definitely need to keep them in a tank on their own - whoever kept them with trigger fish condemned their animals to a certain death. Trigger fish are a main predator of nautilus and will kill each and every one in the tank.
My overall advice is, get something else. Nautiluses are beautiful, yes, but they are also quite boring aquarium animals as they are sedentary 90 % of the time. They need to be kept in the dark, so they don't go blind (this happens a lot with animals from pet stores), they need very good water quality and constant monitoring. If you are just starting with reef tanks, I would recommend something more interactive and enjoyable to start with.
Hope this helps!
Robyn.