Why do I have to do so many water changes?
Imagine, if you will, this nightmare scenario:
You’ve been trapped in a box. All of the liquid and solid waste that would normally be excreted from your body is converted to gasses that remain inside the box. Oxygen is constantly pumped into the box so you don’t suffocate but none of the waste gas is allowed to escape. This is your life now. This is what you breathe into your lungs in order to survive.
How long do you think you could live under these conditions? How long would you even want to live under these conditions?
If none of that sounds appealing to you then you can imagine that fish don’t love it either. When you change a percentage of your tank/pond water you’re effectively flushing the toilet for them. This is why is is so important to either siphon or vacuum the sludge that collects in the bottom of tanks and ponds. If solids aren’t removed with the contaminated water, they will continue to contaminate the clean water it is replaced with.
Of course, the reality is a bit more complex than that analogy but the bottom line is that without water changes most of us have no really effective way to remove the final result of aerobic biofiltration: nitrate.
To understand how the nitrate gets into the water we’ll have to discuss the nitrogen cycle. Nitrogen exists in many chemical forms. Nitrogen gas notably makes up the more than 75% of the earth’s atmosphere and thus is quite common and also not really toxic in any way. There is actually quite a lot of dissolved nitrogen gas in water but it is mostly inert. It isn’t in a chemical form that most organisms can use although some specialized bacteria like those that live in the roots of legumes which are capable of converting nitrogen gas into nitrogen compounds that plants and algae can use. The nitrogen that we’re worried about enters the tank or pond as ammonia from fish waste and the decomposition of uneaten food. Ammonia is very toxic to fish and this is where biofiltration comes in. Inside your filter is a massive army of beneficial bacteria. Your fish rely on these bacteria to consume the ammonia and convert it gradually into less toxic compounds. First, one group of bacteria convert ammonia to nitrite. At this point it is still quite toxic but another group of bacteria help out by eating the nitrite and converting it to nitrate. Nitrate isn’t all that toxic in the big scheme of things so this is a dramatic improvement over ammonia and nitrite but there is a problem. In order to complete the nitrogen cycle we need to find a way to convert the nitrate that is now building up in our water back into nice, inert, nitrogen gas. In nature this is done by various anaerobic bacteria in very low oxygen conditions and this is where the problem lies for most of us. We need aerobic conditions (plenty of oxygen) in order for our fish to breath and all the other biofiltration we rely on also requires aerobic conditions.
So what can be done?
Well, there are technological solutions such as sulfur pellet reactors ,which are capable of at fostering the conditions necessary for denitrifying bacteria to convert nitrates and nitrites to nitrogen gas. For many commercial reef systems, sulfur reactors or biopellet reactors are an important part of keeping fish and invertebrates happy and healthy but all equipment comes with costs and associated risks.
For the rest of us there are regular water changes. For a simple freshwater environment this can be quite painless and even in hobby reef systems this is the most common and accessible method of removing excess nitrates and adding necessary minerals.
How regular? This is a one size fits all question with a pretty specific answer depending on some conditions such as:
fish load
food load
pH
mineral content (calcium, carbonate, etc.)
Having said that, a good rule of thumb in tanks is one water change per week of 30% tank volume. This rule works well for small decorative ponds and water gardens as well but larger ponds can reduce frequency and volume percentage to a general rule of one water change of about 20% volume per month. It is important to note that brand new tanks and ponds should not have any water changes or filter service in the first 4-6 weeks after initial addition of fish. It is normal and good in the first week to see a white cloudiness to the water. This is beneficial bacteria blooming in order to colonize the surfaces of the tank/pond substrate and filter. Once biofiltration has been established you should start with a regular water change schedule and use a test kit to record changes in water chemistry each time. The data you collect will allow you to make informed decisions about when to change water more or less often. You may notice that in a large pond with few fish that you feed only once every other day the water needs changing less often than a crowded 10 gallon tank that is fed morning and night each day.
There is another reason to change water, even without elevated nitrate and this is pH. Over time, biological processes will make your water more and more acidic. This is fine to a point and if you aren’t checking the pH, you might not notice it happening until it is too late. At around 6.0 (in freshwater) the acidity will cause much of your population of beneficial bacteria to die off. This can happen rapidly and when it does the toxic nitrogen compounds we discussed earlier will begin to rapidly build up in the water. It isn’t uncommon in this scenario to go to sleep to a seemingly healthy tank/pond and wake up to a piscine massacre. In a reef environment, the effects of pH deviations can be much more obvious with quite small fluctuations because of how invertebrates such as corals react to sudden changes in water chemistry. When we replace water we do a lot to stabilize the pH by removing acidic water and replacing it with almost certainly more alkaline water even without adding any special buffers, etc.
Key Points
water changes are required for most of us to keep fish alive long-term
water testing is an important tool to tell us when and how much water to change
lack of water changes leads to dangerous pH conditions and toxic nitrogen accumulation
