Thursday, March 28, 2013

Fishkeeper 101: Introduction to Freshwater Chemistry

Anyone who's ever tried to keep fish more than idly has become aware that there's a number of chemical concerns when it comes to maintaining your fish in a healthy state. In general, the chemistry of fish tanks can best be explained in terms of two fields: additives and tankwater conditions.

Tankwater Conditions

Testing of tankwater conditions is best conducted with liquid drop test kits, such as those produced by Hagen under their Nutrafin line. I myself use the Nutrafin Master Test Kit.


Chlorine is not a chemical you can, or even should, test for. If you're on municipal water which contains chlorine, it can be removed after several days of sitting in an open vessel other than the fish tank, or, ideally, with the use of a dechlorinating tapwater conditioner (discussed below).

Chlorine is extremely harmful to fish - as a free iron, it burns gill tissue in the same way that chlorine gas burns lung tissue in humans. These burns are untreatable, never heal (only scarring), and are typically fatal.

Oxygen and Carbon Dioxide

Oxygen and Carbon Dioxide are the respiratory gasses used and given off by fish, respectively, and both are present in the water in solution. Neither can be readily tested for, but if your fish are showing a gasping or gulping behaviour, you should immediately add an air pump with bubbler to increase the oxygen content.

Oxygen is introduced to the aquarium through surface movement - a sufficient amount of movement from a Hang On Back filter can suffice for many tanks. Oxygen is also introduced by live plants, who convert carbon dioxide to oxygen when sufficiently stimulated. I'll talk more about carbon dioxide, live plants, and lighting in a future instalment of Fishkeeper 101.

pH, the Power of Hydrogen

The pH test is the most often administered test in both professional and amateur fishkeeping - it is the first most variable water chemistry condition after the ammonia level. pH testing methods vary depending on the test in question and you should always follow the supplied direction.

pH boundaries are determined by your fish stock, but as a general rule, a stable pH is always preferable to a dynamic one - you want your pH to change less than 0.2 per day, and 0.2 per day is a large change. pH can be adjusted in a variety of ways - my personal favourite method of lowering pH is to filter over (pure) peat moss), whereas I usually raise it using a commercial conditioner for that purpose. Vinegar and Baking Soda can be used in a pinch, but such changes are highly variable and not preferred.

Ammonia, Nitrites, and Nitrates

Ammonia is another major fish killer - I've explained before how the nitrogen cycle works. You should test your ammonia and nitrate levels weekly on the day before your scheduled water change - an elevated level of Ammonia (basically, anything higher than zero) indicates either that something has happened with your cycle  or that your current biological filtration is insufficient for your fish load - add some zeolite to your filter media or consider upgrading to a larger filtration unit or even to canister filtration. A high nitrate level (over 20 mg/L) is considered dangerous - your water change should be heavier. If you consistently get high nitrate levels on test day, add more live plants to your tank. Nitrate is a nutrient used by live plants to manage their growth. Nitrites really only need to be tested during cycle monitoring.

Keeping nitrate levels under control with live plants also helps keep awfuchs growth to within acceptable levels.


Salinity is rarely tested - testing it requires a hydrometer and presupposes that you likely added salt to your tank. For most freshwater fish, zero salinity is perfectly tolerable. Some fishkeeper, including myself, include a small amount of aquarium salt in even freshwater setups to maintain electrolytic balance. Everything I've seen on the practice has been apocryphal but I've seen some very promising results.


There are actually two types of water hardness that are relevant to fishkeeping - general hardness, and carbonate hardness. Generally speaking, a majority of fish prefer their water as soft as possible, but always research the needs of your potential pets before purchase. Like pH, hardness can be adjusted - peat filtration lowers Carbonate Hardness, and general hardness with it.

In tanks with relatively high pHs, such as African Cichlids or Livebearer tanks, and even in Goldfish tanks, an elevated Carbonate Hardness can help "buffer" the pH from dropping over time. The two are separate tests.


The calcium test is closely related to the Hardness tests - with both measures, you can obtain the ratio of magnesiun to calcium, which is useful for keeping marine fish. However, some freshwater fish need a certain amount of calcium in the water for healthy growth and development - especially invertebrates  Tanks with large-growing fish or invertebrates should have their calcium monitored and kept above 5 mg/L.

With the hagen calcium test kit, this testing accuracy can be achieved by testing on four times the amount of water called for, and dividing the results obtained by 4. Having said that, this test is really only necessary in Saltwater except as a cursory inspection every few months or so.


Phosphate testing is important in freshwater terms as allowing a vector of control over algae growth - in general, you want to have less than 1 mg/L to avoid algae getting too strong a foothold. This can be monitored with a simple test and growth in water phosphate levels is usually minimal if you feed low-phosphorous staple diets and keep up with your water changes. No data suggests a link between Phosphates and toxicity in fish.

In extreme cases, phosphate remover packets can be added to standard hang-on-back filters as additional media and used for up to two months.


There are actually two forms of iron you want to keep an eye on in your aquarium - free iron, also known as unchelated iron, which is toxic at levels above 0.3 mg/L. It's best controlled with water changes. Hagen's water conditioner, Aqua Plus, can be used in a triple-dose after the water change to chelate the remaining iron.

Chelated Iron, however, is more of a balancing act. If you have live plants, it should generally be present between 0.25 and 0.5 mg/L as an essential nutrient for plant health. The precise amount varies depending on plants, and I'll explain that in a future Fishkeeper 101 post.

Water Additives

Water Conditioner

Nobody should ever be without a good tapwater conditioner such as Aqua Plus. You'll use it to great extent if you're doing your water changes regularly, and if you're like me, you'll also likely apply it every time you acclimate fish. I use so much of it that I almost always go for the largest available bottle as value for money. A proper tapwater conditioner removes chlorine and chloramines, neutralizes heavy metals such as iron and lead, and, ideally, has a distressing agent that stimulates slime coat production in fish.

Cyclic Boosters

These products, like prime and cycle, often seem like a good idea. They're essentially bottled nitrifying bacteria. In my experience, their use does nothing to augment the speeding of your cycle - if they came with your tank kit, feel free to use it, but don't buy it if you don't have to.

Fish Waste Liquifiers

Some people use these (hagen makes one called Waste Control) to break up larger amounts of solid waste - such as produced by plecos or grown goldfish - which is then subject to uptake by the filter. I prefer to do a good strong gravel vacuuming.

Water Clarifiers

These products come in a variety of forms, and are too broad to discuss in full here. Some are toxic and need to be dosed carefully - what's more, there's no water clarity problem that can't be managed with polywool in the filter and phosphate/nitrate balance management. I classify these products as "quick fixes". They aren't inherently bad, and I'll use them for tanks I'm caring for for other people, but for my own tanks, I'm patient enough to do things the long, cheap way.


These products also come in a wide variety. A staple fertalizer, like Nutrafin Plant Gro, is actually very useful to have around as it's one of few good ways to bolster the chelated iron level without raising the free iron level. Heavily planted tanks need these, and most other planted tanks could still stand to see them.

Again, I'll go into further depth with these on a future post.

pH Adjusters and Buffers

These are quite common, very useful tools for those who want to maintain species that they aren't ideal for. For example, in my present situation, I would need to use pH adjusters to keep African Cichlids or Livebearers happy.

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