Introduction to essential Chemicals for a 75+ Gallon Fish Tank

Maintaining a large aquarium is part of pet ownership and part science experiment. Mimicking an ideal natural aquatic environment in your home takes some care to ensure the chemistry doesn’t get out of balance.

Although it may seem daunting at first, maintaining ideal chemistry gets easier with time. After you learn which tests to run on a regular basis, it becomes second nature. Adjusting most chemical levels is as simple as adding a specific adjustment agent and retesting to ensure levels are within the tolerable range.

The main chemicals (or problems that can be adjusted via chemical means) you’ll want to keep an eye on when caring for your fish tank are:

  • Chlorine
  • Algae
  • pH
  • Ammonia Levels and the Nitrogen Cycle
  • Water Softness


Chlorine has no place in the natural environment of a fish, yet is found in abundance in tap water. Although bottles of aquarium water are available, they are quite expensive – not to mention extremely difficult to transport when dealing with 75+ gallon tanks. Most of us will want to mix our own water to enjoy the cost savings and convenience factors, and since tap water generally has high levels of chlorine, this is the first place you’ll want to start.

Chlorine neutralizing agents are typically added to the water upon freshly filling the tank. It must be given 24 hours or more to fully work before introducing fish. Once the chlorine is eliminated, it won’t come back, but any chlorine in future fills or top-offs must be neutralized before it’s added to the solution. Always neutralize chlorine before adding it a tank with fish in it – never attempt to neutralize chlorine with fish in the water.


Algae accumulation makes it difficult to see in the tank, and excessive algae are not optimal for fish health. This is one area where fish owners are urged to steer clear of chemicals. You’ll see many chemicals on the market which neutralize or destroy algae, but there are more direct methods of cleaning the water and controlling growth. A chemical additive for this purpose only places additional stress on your fish population.

Read more here: How to lower pH in Fish Tank.

pH Levels

The percentage of hydrogen (or pH) of your water indicates the level of alkalinity or acidity of the tank environment. This is one area that’s best to consult a book specific to your type of fish, as there’s no set pH level ideal for all fish. However, most people tend to overplay the importance of pH. The truth is, natural environments may differ greatly in pH, and achieving optimal levels is not necessary for most fish types. Regardless, if your fish requires a certain pH level, it’s ideal to test it often and adjust accordingly.

Ammonia Levels and the Nitrogen Cycle

Nitrate, nitrite, and ammonia are all chemically related compounds that are detrimental to the health of your fish. As ammonia is produced, it’s converted into nitrite, and then into nitrate. The confines of a fish tank don’t provide enough volume (even when dealing with huge tanks) to dilute these substances substantially enough to prevent dangerous levels from accumulating. There are chemical means of removing these compounds, but they can cause problems in regards to the health of your fish.

It’s for this reason that fish tank water should be partially flushed on a regular basis. Weekly, you’ll want to remove and replace 10% of the water. Once a month, a deeper flush should be performed, removing and replacing 25% of the tank water.

This flushing practice will ensure ammonia, nitrate, and nitrite are all kept to a minimum. Although this practice is usually enough to keep these chemicals under control, it’s a good idea to purchase a set of test strips designed to measure the levels of these compounds. This will allow you to identify and address any problems that a basic flush doesn’t solve.

Read more about it here: How to Cycle a Fish Tank.

Water Softness

The “hardness” of water refers to the amount of dissolved minerals present in the water. Fish aren’t extremely picky about hard vs. soft water, but a high extreme of either hardness or softness can affect the pH levels. Although pH itself isn’t usually important if it stays roughly the same over time, unstable pH levels can shock your fish from the sudden change.

For this reason, it’s important to maintain balance water softness to ensure the pH doesn’t fluctuate suddenly and harm your fish. You can test for water hardness, and adjust as needed. Soft water can be adjusted by the addition of minerals, and hard water can be diluted by the addition of a few gallons of distilled water.

Maintaining Ideal Chemistry

Armed with this knowledge, you’ll know what to expect while maintaining your fish tank. All of these values can be measured with test strips designed specifically for each chemical, or you may wish to pick up a variety pack of strips in order to cover all bases. A weekly or biweekly test is recommended at first. After 6 months, if your tank seems to be stable, you can reduce your measuring practices to less frequent intervals – although it will still be important to check these metrics on a regular basis to ensure nothing runs out of control.


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