A guide to the various waters:

Distilled Water is water that has been boiled to evaporation (steam) and then condensed back into water. This process is usually carried out a few times (evaporate and condense and then evaporate and condense again). By evaporating the water you leave any impurities (100% of salts, heavy metals, other nasties) behind and what you end up with is 100% H2O. This is the purest water you can acquire. Distilled water is good for misting terrariums because it won’t leave a mark on the glass when it evaporates (the marks left on the glass by other waters are caused by dissolved salts, which are not present in distilled water). If you want to use distilled water in aquariums or frog water bowls, etc, you need to replace the lost minerals in it by using a salts mix (such as Holtfreter’s – an exact recipe for a mixture of a few mineral salts) or by using a commercial product like R/O-rite or Electro-rite, both intended for Reverse Osmosis water but equally suitable for distilled water.

Reverse Osmosis Water (R/O) is not as pure as distilled water but pretty close – reverse osmosis basically filters out anything dissolved in the water. People buy reverse osmosis systems at aquarium stores so they can make up water with precise water chemistry for applications like salt water aquariums or dart frog enclosures. Distilled water is equally good but obviously it’s not as easy to distil your own water on a useful scale. If you plan to use R/O water in an aquarium or a water bowl then make use of a product like R/O-rite or Electro-rite to replace the lost salts necessary for normal water chemistry.

“Purified Water” is usually water that has been filtered by various different methods, or even distilled, and had minerals added for taste (your taste, not for animals). There’s not much to recommend using it instead of tap water that has been treated with a water conditioner, because while tap water will have extra things in it, “purified water” will be lacking in normal dissolved salts that animals need. I take a very dim view of “purified water” – think Dasani and other products – because it seems like an excuse to charge a lot of money for a bottle of filtered tap water.

Spring Water – depending on where you live this can mean different things. In Europe there are strict purity standards on what you can and can’t call a Spring Water. I’m not sure how it works in the US and elsewhere. Spring water is usually water drawn from an aquifer (natural collection of water in the ground, often mind boggling in volume).

Mineral Water – again I’m not sure if the same standard applies outside of Europe, but mineral water is similar to spring water except that it meets much higher purity standards. True mineral waters are rare in modern times due to all of the pollution in the atmosphere that goes into ground water when there is rain and snow.

Tap Water – what comes out of your tap really depends on where you live in the world and what your local municipality adds to the water. Tap water in most countries is treated to reduce bacterial content (and other nasties), often pH adjusted (making it less acidic usually, though sometimes less alkaline/basic) and various salts are added to it (usually for pH adjustment). Aluminium sulfate is another salt added to water to remove particles, which improves water clarity (who likes drinking brown water?). Chlorine, a caustic gas, is added to water to kill nasties and also remains in the water for a day or two, thus keeping it “safe” until it comes out of your tap. Chlorine in water will kill fish and can kill tadpoles. You can get rid of it by leaving tap water in a bucket for 24-48 hours. However if your municipality also adds ammonia to the water, the ammonia interacts with the chlorine to create chloramines – combinations of chlorine and ammonia that don’t dissipate from water left in our bucket for a day or two. You may be able to find out from your municipality if they use ammonia. Many municipalites also add fluoride to water because it strengthens tooth enamel. Aside from all of the things I’ve mentioned in tap water so far, tap water also picks up metals (copper, lead, iron, etc) from running through pipes. Therefore, if you wish to use tap water, purchase a water conditioning product intended for aquariums, such as Tetra Aquasafe or Amquel. These remove chlorine, ammonia/ammonium and chloramines from water, as well as rendering any dissolved metals inert. The tap water is then safe to use with your amphibians.

Well Water – this source of water usually has a lot of extra dissolved salts and metals in it, and unless you have your well water tested regularly, it’s best to stay away from it due to the often very high pH and metal content.

Dionized Water – Deionization is a completely different process to Reverse Osmosis and requires different equipment. It removes salts from water and relies on their ionic nature for their removal. It leaves behind any species that are not ionic in nature, including organic molecules and even bacteria.

So now you know.