Hi bodasafa,
Some species of fish temporarily lose their colour as a protection strategy. This can happen if the fish are stressed out for some reason, like if they are new to the tank. One such fish is the blue orange fin loach. If this fish is stressed, he's a grey silver (almost black) colour, when it settles down, the beautiful blue colour returns. Also, in some species such as for instance the blueberry tetra, they are somehow coloured (perhaps with some sort of harmless colour die) when they are bred and sold. These will eventually lose their colour and turn white. This is not however, the case with neon tetras.
The neon tetras may be a little confusing to talk about because they may actually be a tetra of one of several related families of tetras including the false neon tetra (which has very similar colours to the neon tetra and hence it's "false" name), the cardinal tetra which is a two tone silver and red colour along the length of it's body right to the tail, and also sometimes mistakenly called a neon tetra. The actual neon tetra (Paracheirodon Innesi) has a blue-silver colour along the top and also along 40% of the side of it's body. Then it has red colour beginning at it's underbody fin, and going right to it's tail. This information is intended to help you identfy the kind of tetra you have for future reference if needed.
The good news is that regardless of the tetra that you have, they all come from the Brazilian Amazon and therefore prefer generally similar aquarium and water conditions. They like well planted aqauriums (false or real plants it's irrelevant), with some open spaces and low lighting or shadowy conditions. Some people use floating plants to get the shadow effect, but this is not critical. Soft and somewhat acidic water is preferred, but they may tolerate harder more alkaline water if you're careful to maintain a gradual easy change from one extreme to the other (from hard to soft water or vice versa). The temperature range that they prefer is 73 - 81F (23 - 27C) but you must make sure that your sharks will tolerate similar conditions for all things concerned.
Some information in regards to Red-Tailed Black Sharks, Ruby Shark (also known as the Rainbow or Red-Finned Shark). This species of shark will not show off their brightest colour if the water and/or aquarium conditions are not right for them. They will become a sort of darkish grey and lose some of the red off their tail fin as well. This sounds like it may be your problem.
These sharks like aquariums with shelters or caves, and a fine-grained substratum and soft to medium-hard water. They prefer a water temperature range of 77- 79F (22-26C) and higher for breeding.
You should as a precaution however, look up diseases that may cause a discoloration of both these species, and also try to speak to a knowledgeable person at your local fish supply. If it is a disease, then the fish supply may know of remedies that will solve your problem, or they may have some sound advice to at least relieve your worry. Please make sure that the person you speak to is not just "talking out their hat" (to be polite), more information is always better than less. You might (if you believe that disease is present) talk to your local vetrinarian. Some Veterinarians are well versed on the subject of aquaria invertibrates (fish) and disease etc.
Check your water first! Use a test kit that is less than 3 months old (you may have owned it for 3 months, but it may have sat on the shelf at the store for 9 months or more) to test the water for Amonia, Nitrate, Nitirite, Total Hardness (GH), Total Alkalinity (KH), and PH level as well. Once you're certain that your water is right, then move on from there.
It has been said that if you keep good water the fish will keep themselves!
This is alot of information to digest in one go to be sure. Read all you can on the species you have, and then make adjustments as you see fit. There are a lot of good books available on the subject of fresh-water tropical fish, and I've found that having at least one good reference has saved me hundreds of dollars and hours (if not days) of worry, to say nothing of all oif the lives saved in my little 40 gallon tank.
Good luck.
Sheloare
