(Continued from Beyond H2O Part I--Intro, Dechlorination/Impurities, pH, KH, GH & Salinity)
As we all know, fish, like other animals, breath in oxygen and give off carbon dioxide. They must extract oxygen from the water in its dissolved form by way of their gills. Honestly, however, as a beginner I would not even worry about oxygen content in the water.
If you have a properly running filter, which you need for waste removal (described below), you already have enough circulation to aerate the water, even if you have a tightly-fitting hood and no airstone. If the tank is so overcrowded that low oxygen levels actually get to be a problem, you would actually have much more serious problems to deal with, not just problems with oxygen. One of these, another cause of gasping, has little to do with the amount of dissolved oxygen in the water--it is ammonia poisoning, which is fully described below.
The only other dissolved gas that beginners ever really concern themselves with is carbon dioxide, CO2. Again, this isn't really anything to worry about in the average tank, but I only bring it up for two reasons. The first is that it is one of the gases that is highly compressed in tap water, but which comes out of solution when you put it in the tank, leading to little air bubbles on the glass and objects, sometimes on the fish. In the large majority of cases, this is not a problem. Sometimes, people notice that the pH of the water will rise slightly from when they first measure it straight out of the tap to after it has been sitting out for a while. This is usually just an effect of CO2 coming out of solution as a free gas (in solution, it is in equilibrium with carbonic acid). Again, really not a big concern for the average tank, since you always should be doing regular percentage changes anyway, never 100% at once.
The last thing to mention about dissolved CO2 has to do with the fact that plants do use this as a nutrient. As such, it is often injected into the heavily planted tank to help the plants grow better (but first and foremost, good lighting should be your first concern for plants--read this article on planted tanks for more info). CO2 retention is also helped along by reducing splashing and surface agitation.
There are no regularly sold tests for either of these gases in aquarium water. Water can be saturated with BOTH gases at the same time.
So far, because of the order of progression of this article, I've only mentioned the things that can affect the water chemistry before it even goes in the aquarium! But some of the most toxic compounds can actually come from the fish themselves, in the form of their own waste. In Nature, these toxic waste compounds are quickly diluted away in fast-flowing rivers or huge volumes of water.
The aquarium, however, is a semi-closed environment where they will just get more and more concentrated. Just "topping off" water alone (adding just enough to make up for that lost by evaporation) is NOT a replacement for water changes and establishment of balanced biofiltration. Only pure water can evaporate away; this leaves behind all the solutes, gradually concentrating them over time and making tank conditions awful. Water changes should be done regularly for the overall health of the tank--I recommend every 20-30% every 1-2 weeks,
depending on the size and population.
Back to this concept of waste accumulation--the other water quality concept (aside from regular water changes) VERY important for beginners to understand, is the idea of "cycling" a new tank. A new tank is not a safe place to fill with fish, a concept more fully explored in my article, Choosing a Filter Part I--How Filters Work. In short, the major component of fish waste is ammonia, which is highly toxic at any level above 0 ppm (parts per million). If exposed to this poison enough, the fish will eventually poison itself to death or get so stressed that its immunity can't work to fight disease. The idea behind cycling a new tank is to give it enough time to establish the proper "good" bacteria to grow in the filter. These bacteria will decompose ammonia to steadily less poisonous forms of nitrite (also toxic above 0, but not as much so) and nitrate (for all intents and purposes, nontoxic to fish at moderate levels, but will encourage algae blooms; can be kept down through water changes).
How do you establish these bacteria so that you can start converting ammonia? First off, don't get fish right away when you get a new tank. You will get details on how to use commercial pure, unscented ammonia (which you can buy at most drug stores and supermarkets as a cleaning solution) to cycle the aquarium before you even get your first fish, all the details are within the article. The idea of fishless cycling is to save your fish when you do get them from as much stress and disease as possible; so many times, an uncycled tank is responsible for the death of fish in beginner tanks during the first 2-3 weeks of setup. Ammonia and nitrites accumulate, but because you can't see or really smell them, new fish owners have no idea what's going on.
One tip for speeding up the cycling process (though this will take time regardless) is to add the "good" bacteria directly. This can be done by way of adding old, used filter material from another healthy and established tank. Alternatively, there is a product on the market by Marineland today called "BioSpira", and this is expensive but works very well (in my opinion, MUCH better than other products like "Cycle" which claim to do the same thing, but really don't work well at all). Products such as "Ammo-Lock" (Aquarium Pharmaceuticals) can bind up toxic ammonia in emergency situations, but should not be used on a regular basis because they will interfere with the natural cycling process. Tests for ammonia will still show false positives with use of "Ammo-Lock".
I know it all sounds very overwhelming, but really I've put in a nutshell the most important water chemistry points you will need to know for maintaining fish health in the aquarium.
In summary, here are tests which I consider most important to have as a beginner with your first FW tank:
Those are the bare minimums--such a short list after so many lengthy definitions! But I wanted to only include the most important first, if you are pressed to choose between "necessary" vs. "good but optional" tests for beginners. Ammonia and nitrite levels are vital to know in any tank, especially a new one in the process of cycling, or one which is experiencing disease problems. Anyone experienced with fish health will tell you that 9 times out of 10, one of those values is elevated in tanks with diseases, and most respectable sources of help when you do get disease will ask you those values right away. Ideally, anytime you have fish, both those values should be at 0, but the catch-22 is that during the early stages of cycling, both will by definition have to rise. This is the reason I recommend fishless cycling, so that there are no fish in the tank during this period to experience this stress.
Those 2 are the absolutely necessary tests for beginners. After that, in order of importance:
pH (just to have some idea, but as mentioned above, it's more for your own knowledge, because trying to modify it is usually not a good idea)
Nitrate (not toxic to fish, but will give you a good sense of whether or not your tank is done cycling, because all cycled tanks will have a low level of nitrates once they are established; also, they can sometimes give an indication of why algae problems are occuring, and can be used as an indirect reference for other organic by-products of fish waste)
For saltwater, all of the above should be purchased as necessities (since SW animals tend to be more sensitive), plus:
Hydrometer (for measuring specific gravity)
Optional tests for marine reef aquaria (which have corals), in order of importance, includes tests for:
Copper (since it is toxic to invertebrates--only recommended for cases where RO/DI water is not used and copper poisoning is suspected)
Phosphate (along with nitrates, another big indicator of what algal blooms are feeding off of)
Other Trace Elements (most of these are infrequently used and VERY optional, calcium is really the big trace element to measure for)