Pellets are balanced, nutritous food for fish that will eat them. Think of them as the equivalent of food astronauts eat in space.. a balanced meal packed into a mash of solid food that don't look like food. However, do note that there are many different quality of pellets available in the market, some cheap ones are just made like hard biscuits with a little bit of ingredients mixed in. Personally, I know of at least one person who can differentiate the good pellets from the cheap and maybe fake ones just by tasteing. Yes, can you believe it? This guy actually chews them in his mouth and taste them before spitting them out! I have personally tested him using different pellets with his eyes in blindfold, and he could tell which are the lousy ones and which are the expensive ones! He told me I could do it too if I taste enough varieties of pellets but I just don't feel good about putting petfood in my mouth...
There is no governing bodies on petfood in most countries that manufacture petfood, so generally the nutrition analysis on the label can be accurate, half accurate, or even totally faked. One safe way to get good pellets is to get them from a reputable brand. These brands have to manufacture the pellets to a consistent quality so as to safeguard their high standings in the aquaria market.
Price is not to be used to judge the quality of pellets although those made by good brands are usually more expensive. A lot of medium quality pellets are made by the same manufacturers, which are labelled by different companies and sold at different prices for essentially the same product. There are still differences because them they can be stored differently and packed differently. Pellets can go bad easily so bad storage can cause deteriotion. Different packaging also makes a difference, whether they are sealed in vacuum (vacuum packed), sealed in airtight seals or just plastic shrink wrap.
While pellets can be the main course for most fish (many predatory fish will never eat pellets and can starve to death if you are insistent), it is still good to give your fish an occasional treat with other food like bloodworms and prawn meat etc so as to give them a more balanced nutrition.
Floating and Sinking
There are two types of pellets, they are floating and sinking pellets. Floating pellets float on the surface and is suitable for most fish. Sinking pellets are best suited for bottom feeders like corys, plecos and most catfish. However, most bottom feeders will also take floating pellets if they are hugry enough and are not outcompeted by other mid-water column and surface dwelling fish (However this may not be good for them because eating food on the surface makes them take in air, with they cannot expel effectively).
There are pellets made with natural and artificial colour enhancers. Ingredients like carotene and astaxantine is natural. Artificial colour enhancers are labelled as something like 'Colour enhancer 123.4'. Natural colour enhancers help the body produce the colour by the fish, so the colour is enhanced in all the right places and make the fish look better. Artificial colour enhancers are the colour pigments themselves, so the effect is very natural. These artificial colours generally show up most in the fins and the parts where the fins join the body. Because they are artificial colour, they do not match with what the fish's colour should be, causing the fish to look unnatural. Not only does artificial colour enhancers cause unnatural colouring, they are also damaging to the fish's livers in the long term.
If the label of a packaging says something like 'delicious and colour enhancing' but does not state whether it is artificial or natural colour enhancers, you can test it by putting some into a while plastic disposable cup and add warm water. If the water turns very reddish, it is artificial colouring.
1: Very slight colouring is ok. Sometimes a little colouring is used to make the food look nice. Most of our human food have artificial colour too.
2. A fish that look like colour faded is not suffering from a lack of colour enhancers. It is because it is stressed from sickness or there is something wrong with the water. Bring out your test kits!
Also buy as lttle quantity of pellets as possible (without having them run out in a week). It is not that difficult to buy new pellets if you go to fish shops regulary or if there are fish shops near you. This is because pellets can go bad quickly once they get opened! Don't save a few cents and then having to discard most of it away.
Another reason is that you can buy a different brand of pellets everytime you run out of them. You may come across a pellets that your fish particulary like!
Most fish and invertebrates will readily feed on pellets, as long as they are the right size to fit in their mouths. Some carnivorous fish have to be converted to eat pellets because they do not readily recognise pellets as food.