Over time, aquarium water gets dirtier and depleted of essential elements, and it will never improve on its own. Because water quality is so important, and it always gets worse over time, starting out with proper water is crucial to aquarium success. This is true when starting a new tank, doing water changes, and topping off your aquarium when water evaporates.

Here are instructions for how to prepare water for the four main types of aquariums: Freshwater Fish Tanks, Planted Aquariums, Saltwater Fish Tanks, and Reef Aquariums.

In most freshwater aquariums, the best way to prepare water is to use tap water with a water conditioner. Tap water contains chlorine and ammonia, and a high-quality water conditioner will remove both toxic compounds, making the water safe for fish. Our favorite water conditioner is Seachem Prime. You should add the water conditioner to all water used for water changes and top-offs.

Many other undesirable compounds in tap water are not toxic to fish but will cause excessive algae. With the low light levels that most fish-only tanks receive, this is not a problem. If you are experiencing algae problems, it may be worth switching to reverse osmosis (RO) water to help solve these issues.

In planted aquariums, the emphasis is on keeping live plants. In planted aquariums, there is more light than in fish-only tanks, and the plants use minerals in the water to grow. These things combined make reverse osmosis water combined with a remineralizer the best choice for planted tanks.

Tap water often contains extra nutrients, such as phosphate and silica, that can cause algal blooms, and it often lacks the calcium, magnesium, and other elements that plants need to grow. This is not to say that tap water can't work, only that the quality of tap water varies widely, and without knowing exactly what is in your tap water, the path with the greatest chance of success is RO water with a remineralizer.

You can make your own RO water with an RO system from SpectraPure, AquaticLife, or Ice Cap, or you can purchase RO water from your local fish store or grocery store. RO water is extremely pure and lacks many of the minerals plants need. You can add minerals to RO water using Brightwell Aquatics Remineraliz, or use a more natural solution such as Sera Alder Cones.

You only need to add a remineralizer when you set up an aquarium or do a water change, not when you top off the tank. This is because only water leaves the aquarium through evaporation, while the minerals remain in the water. Top-offs with pure RO water are ideal.

Saltwater fish tanks are aquariums with saltwater fish, no corals, and at most a few invertebrates. In saltwater fish tanks, tap water can work, but most aquarists will see a significant reduction in nuisance algae when using RO/DI (deionized RO) water. RO/DI water is purer than RO water because it goes through an extra filter stage, making it even cleaner. You can use an SpectraPure, AquaticLife, or Ice Cap to make RO/DI water or purchase the water from your local fish store.

With saltwater aquariums, you will need to mix salt in with your water before starting your tank or performing a water change. When topping off, you do not want to add saltwater. This is because the water in the aquarium evaporates, but the salt stays. Topping off with saltwater will slowly raise your tank's salinity,, while topping off with RO/DI water will keep it stable.

When selecting a salt for your saltwater fish tank, choose the most affordable salt. The extra minerals in expensive salts are important for reef aquariums but do not matter for fish. Our favorite fish salt is Two Little Fishies AccuraSea1.

A reef aquarium has both saltwater fish and corals, and for reef aquariums you want to use RO/DI water and high-quality salt with trace elements.

There are two important reasons why you want to use RO/DI water with all reef aquariums.

1. Reef aquariums have very intense lights, and the more light you have, the more algae you will have. RO/DI water is so clean that nuisance algae can't thrive in it—even under intense light.

2. Corals are very sensitive to many pollutants in the water supply. An RO/DI will get you water as pure as possible.

Corals rely heavily on trace elements in the water. A high-quality salt mix will have these trace elements.

When selecting a salt for your reef aquarium, there are many good options. Our favorite option is Two Little Fishies AccuraSea1. AccuraSea1 is a bit more expensive, but we believe it is the highest quality salt on the market and worth the extra cost.

We also recommend Two Little Fishies AccuraSea1 salt for its packaging. The 50-gallon box has 10 individually packaged 5-gallon salt mixes that solve the problem that many aquarists run into when they do not use all the salt in a container. When an entire container of salt is not used all at once, you risk making salt with either too many or too few trace elements. When a bucket of salt sits for a long time, the heavier elements in the salt mix tend to settle at the bottom of the bucket, while the lighter elements end up on top (a process known as stratification). As a result, the parameters of your saltwater vary from one water change to the next.

A great RO/DI system choice is the AquaticLife Twist-In Compact 4-Stage RO/DI Unit, 100 GPD. Or, you can purchase water from your local fish store. Just like with saltwater fish tanks, you only want to use saltwater when starting an aquarium or doing water changes. Top-off water should be pure RO/DI water.

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