I think this is a common question for people brand new to keeping aquariums. It’s easy to think that fish just need water, why do we have to keep changing that water? Just like a water bowl for a dog, we change the water to keep it clean, remove things we don’t want, and supply fresh clean water. This post will talk about the many benefits of water changes!
Benefits
We’ll start with the benefits. A marine aquarium is a lot more complicated than a freshwater aquarium. The salt we use to create our water is not only salt but also a plethora of minerals and elements that create a perfect water chemistry. Many elements get used up by various invertebrates, clams, algae, corals, perhaps even some fish. It’s important to maintain the levels of various elements and minerals and an easy way is by making water changes.
Another benefit is the removal of certain by products like ammonia or nitrate. Especially in a newly set up tank, ammonia spikes can happen. In order to reduce the levels we do a water change, usually a large one. For nitrate a series of small, regular water changes is usually enough to keep it in check.
How to do a water change
Water change is simply removing old water and replacing with new. This is easily done with a small container like a five gallon bucket. We also need something to remove the water with. You can use flexible hose or buy an aquarium gravel vacuum. Some of the newer gravel vacuums include a hand pump to help prime the siphon which is very useful!
If you are doing a small water change compared to your system volume, the water will be alright unheated. For example, in my 125 gallon display tank (which has a total system volume of approximately 180 gallons) I do tiny water changes a few times a week. I like to keep my water changes small for a couple reasons. First it doesn’t shock my system with huge water chemistry changes. Secondly because I don’t enjoy carrying large amounts of water.
Back to my point – if I only change out two or even three gallons of water, my tank doesn’t really notice a big temperature difference with unheated water. If my tank had only 20 gallons, and I did 2 gallon water changes, that could shock my system more easily by using cold water. In this case I would want to preheat my new saltwater before adding to my system.
Doing a water change is really simple. Remove X amount of water and replace the same amount with new water. If I take out two gallons of old water, I replace with two gallons of new saltwater.
Where to dump old saltwater
This is something I never thought of when I first started. I used to dump my freshwater tank water in the yard but I can’t do that with saltwater (it would kill my grass!). If your old saltwater contains ONLY water – no debris, no sand, no little algae bits, you’re fine to dump it in the tub or a kitchen sink. If there is ANYTHING, and I do mean anything, in the water use the toilet instead. Debris in the water, especially sand, tends to clog up the drains in sinks or tubs. Toilets, however, were designed to not clog up!
My tank is set up in my basement. Next to my laundry room where I have a floor drain in my basement. If my water is clear, I tend to dump my water down there. It’s important to note that wherever you choose to dispose of your saltwater you should always rinse with tap water afterwords. Salt can be very corrosive over time and by doing a quick rinse, you ensure your appliances stay nice.
Downsides
If you have a FO or FOWLR tank there really are no downsides. You’re giving your fish clean water. If you have a reef tank with corals there is a small downside and that is changes in the water chemistry. Most of these changes are for the better actually, but some corals don’t like change. The only downside to a water change is that corals can close up for a day. That’s it. They usually open the next day like nothing ever happened so it is a really, truly, small downside.
Mountain Corals 
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