I get asked this a lot through PMs.. and as I was responding to my most recent, I figured to throw this up here in case someone else has questions.. as it pops up now and again on what type of water conditioner to use.
I would recommend the basic chlorine remover water conditioner as that is all you really need. Regular water changes is what is most important in keeping an aquarium healthy. By doing weekly water changes you are removing the deadly toxins that build up, so all you should have to do is make sure that the chlorine is removed from tap water.
Some brands you may want to look for:
API Stress Coat
API Tapwater Conditioner
Tetra Aqua Safe
Easy Balance with Nitriban
Kent - Ammonia Detox
Marineland Labs - Bio-Safe
Nutrafin Goldfish Bowl Conditioner.
Jungle Lab Ammonia Chloramine Eliminator
Wardley 3 in 1 Water Conditioner
It also should be noted that ALL these types of chlorine/chloramine removing conditioners are Redox Reducers
and thereby are helpful in maintaining a healthy aquatic environment by aiding in the removal oxidizing free radicals from the water (albeit temporary); this why it is perfectly fine to add a full tank dose even if only 25% of the water is changed.
The basic premise of an aquarium water conditioner is to remove chlorine from tap water. Some conditioners such as Prime, Start Right, Novaqua, Stress Coat, or similar is usually after water changes to remove chemicals such as Chlorine; many (not all though) also either naturally stimulate fish slime coats (by adding electrolytes) or add ingredients for slime coat “bandages”.
Also it should be noted that most water conditioners of all types (basic, artificial slime coat replacer, and chloramine/ ammonia remover) have similar properties and ingredients and some of the outrageous claims I have read on the internet such as "Start Right is not safe for marine aquariums while Novaqua is" are without any scientific merit.
Some additives they may offer-
They add a chemical (usually tetrasodium EDTA) that renders any dissolved metals that may be in the water into a harmless type. This is actually a useful additive as tap water quality varies enormously across the world and some people have their tap water coming from a local well which possibly has a low level of iron, copper, or zinc in it. This may be safe for people to drink, but may be too high a level for their aquatic pets.
Nitrate or nitrite removers
Some companies add chemicals which also remove these two troublesome chemicals. This is a useful additive, but a correctly cycled tank, with proper, regular water changes, or a planted aquarium should never require the forced removal of these. Best kept for an emergency.
Slime coat protection
These are various chemicals, some natural, and others not, that claim to add a protective layer around the fish skin. Some aquarists say it may cover the fishes gills and reduce the animals ability to breathe. In any case, the animal is perfectly able to provide its own mucus to cover any possible wounds on its body. Typical chemicals are Allantoin (Comfrey plant extract) or aloe vera.
There are no studies as to how or if aloe vera works or doesn't work with aquatic life. Some people suggest that it may hurt fish. Others say it just doesn't help. And still others say it helps promote healing of fish wounds. The debate will most likely continue until proper studies are done and published.
Some conditioners add a bactericide. The thinking goes along the lines that you've performed a water change and therefore there may be bad bacteria churned up from the substrate. One such chemical added is often formaldehyde. When we go to so much trouble to make sure our aquariums have a healthy bacterial filter, one might wonder why some products may be designed to kill it off.
This is claimed to remove the chemical fluorine. This chemical is often present in tap water in the ionic form fluoride which is used to decrease tooth decay in people. No evidence has been found that fluoride is harmful to aquatic animals at the normal dosages (0.7-1,2ppm) from tap water.
Herbal extracts are getting more popular over the last few years, unfortunately not enough studies have been done as to exactly how or if they work on fish or other aquatic life. These ingredients are thought to be introduced because of the rise of homeopathic "medicine" and other non-scientific approaches to healing.
Electrolytes are formed when a salt is placed into water. This will alter the osmotic pressure of the water and provides a similar effect as adding a quantity of normal everyday salt to the tank water.
Common salt is sometimes added in low amounts to a tank to reduce stress on an aquatic animal moving from one water type to another or to improve the uptake of oxygen by the gills of a fish if the animal is ill. Why you need to buy an expensive variation of common salt is not given. Also not explained is why you need to stress your non-ill fish by altering the water chemistry (osmosic pressure) of the tap water they are probably already used to. Perhaps useful if your animal is ill. But ordinary salt is far cheaper.
It must me mentioned that most common table salt has iodine added to it for human health reasons. Do not add this to your aquarium. Kosher salt, Aquarium salt or sea salt, with no additives, is the preferred treatment if one really needs to use salt in a freshwater aquarium. (When it comes to bettas please note the correct dosage and length of treatment.)
If you just want a complete removal of chlorine/chloramine, ammonia then I recommend:
Kent Ammonia Detox
If you want a complete removal of chlorine/chloramine, ammonia, heavy metals and slime coat protection then I recommend:
For just removal of chlorine, heavy metals and slime coat protection then I recommend:
API Stress Coat
Just remember if you can, see what consists in your tap water- call the local city, or possibly look it up so you know what you need to protect from. If using a well, still get it checked, but if it does not contain chlorine, then you will most likely not have to use a conditioner.
I can go on about different controversies about the additives and such.. but for now, this is it.
I personally have used some of these over the last 17+ years with no ill affects from any of them.