Well out of the 3 Ausrailian Rainbows I bought one died
. It died because of internal paracites.
How do you know this? Fish dying from internal parasites is in fact very rare. Parasites have evolved (generally) not to kill their host. There's no point in that happening, since if their host dies, so do they. There are parasites that eventually kill their host, but not nearly as many as people believe.
The top three reasons fish die are: poor water quality, wrong water chemistry, and wrong tankmates. When fish do get diseases and die, it is 99% of the time because of stress through one or more of these factors. Random diseases suddenly springing out of nowhere are very, VERY, rare, just as with humans. We don't often get lung cancer and heart disease for no reason at all, we get them because of smoking, excessive alcohol, poor diet, lack of exercise, and so on.
Check your nitrites. If you just went from 1 rainbowfish to 4 rainbowfish, then your filter has to now deal with 300% more ammonium. That will take a few days, at least, to happen. Now, rainbows are fairly tough fish, so if you cut back on the food, the inevitable spike in nitrites while the filter adapts will be manageable.
Re: discus. Discus are schooling fish when young, and then form fairly loyal, somewhat territorial pairs once they start breeding. It is absolutely standard practise to keep them in groups of 6 or more outside of breeding, with pairs removed to a spawning tank as and when required. Discus should certainly not be kept in ones or twos otherwise.
Also, the other problem with discus is that they need much warmer water than other tropical fish. Whereas standard tropicals need around 25 C, discus have to be kept at no less than 30 C. If you keep standard tropicals at 30 C, they will eventually die through accelerated metabolism and low oxygen concentrations. If you keep discus at 25 C, they will die from opportunistic bacterial infections, because their metabolism isn't fast enough at this temperature to deal with them. There are some tropical fish that tolerate very warm water, such as angelfish and gouramis, but because these fish will bully the discus and introduce bacterial infections, they cannot be used. Adult cardinal tetras, silver hatchetfish, and rummynose tetras are about the safest compansions for discus. Clown loaches can work well, too.
Most rainbowfish need moderately hard, alkaline water to do well. I wouldn't mix them with tetras and other softwater fish. Instead, consider peaceful livebearers, halfbeaks, glassfish, gobies, and other species that need similar water conditions.
Cheers,
Neale
PS. There's no need to clean the entire tank once a week. A 10-50% water change weekly is fine, with the filter medium being rinsed in a bucket of aquarium water once every month or so. Change filter media as infrequently as possible. While a 10 gallon tank is far too small for rainbowfish (many species get to be 8-15 cm long) anything from a "long" 20 gallons upwards should be fine. They're nice fish, and deserved to be treated well. Properly cared for rainbows are long-lived, and should last at least 6 years.