The level of nitrate acceptable to fish and invertebrates is a subject of much debate. All agree that "lower is better". A nitrate level of 40 ppm (mg/L) or less is recommended for freshwater aquariums. Many marine aquarists prefer to keep nitrate levels as low as possible, especially when keeping invertebrates.

Partial water changes reduce nitrates. Other rechargeable filtration material is also available.

Water with a GH will contain a lot of calcium and/or magnesium. This is great if you are keeping African cichlids or other fish that like hard water. While excessive GH is not dangerous to tropical fish, many species come from water with a GH level of 35-ppm to 90-ppm (2-5° dGH). Most aquatic plants also prefer a GH level of 35-ppm to 90-ppm (2-5° dGH). Excess GH also contributes to white scale build-up on the aquarium lid and light fixture. As aquarium water evaporates calcium and magnesium (and other minerals) are left in the water causing GH to rise over time. Lowering GH (softening) has no effect on pH.

Carbonate Hardness, also known as KH, refers to the concentration of bicarbonate (HCO3-) and carbonate (CO3--) dissolved in water. KH is important in relation to pH. KH, also known as alkalinity or buffering capacity directly affects the ease of making pH adjustments. In freshwater a high KH level makes lowering the pH very difficult. Each time a pH adjuster is added some of the KH is "neutralized". Each consecutive dose reduces the buffering capacity (KH) of the water. The pH will not go down, however, until the "breakpoint" is reached. When the breakpoint is reached the KH is sufficiently reduced to allow the pH to go down. This is why it may take several doses of aquarium pH adjusters to successfully lower the pH. Adjusting the pH is much easier after the KH has been reduced.

Maintaining the pH once adjusted can be difficult with tap water having a high KH level. Every time tap water is added more KH is added to the aquarium. This shifts the water chemistry toward a higher pH level. Additionally, when water evaporates, it leaves all the hardness minerals in the aquarium. Some aquarists use distilled water to replace evaporated water. (Other kinds of bottled water like spring water are usually high in minerals and will increase water hardness.)The best way to manage water hardness levels is the Tap Water Filter. The Tap Water Filter will remove all minerals and pollutants from tap water, permitting easy adjustment of pH and water hardness.

The pH of some synthetic marine salts stabilize below or above the recommended 8.2 to 8.4 pH range. Even though the KH is acceptable, the pH may not be right for the aquarium.

Water with very low KH may experience wide fluctuations in pH because the buffering capacity is low and small changes make big swings. A high KH level indicates that the pH will be very stable. High KH is almost always associated with a high pH.

Carbonate Hardness, also known as KH, refers to the concentration of bicarbonate (HCO3-) and carbonate (CO3--) dissolved in water. KH is important in relation to pH. KH, also known as alkalinity or buffering capacity directly affects the ease of making pH adjustments. In freshwater a high KH level makes lowering the pH very difficult. Each time a pH adjuster is added some of the KH is "neutralized". Each consecutive dose reduces the buffering capacity (KH) of the water. The pH will not go down, however, until the "breakpoint" is reached. When the breakpoint is reached the KH is sufficiently reduced to allow the pH to go down. This is why it may take several doses of aquarium pH adjusters to successfully lower the pH. Adjusting the pH is much easier after the KH has been reduced.

Maintaining the pH once adjusted can be difficult with tap water having a high KH level. Every time tap water is added more KH is added to the aquarium. This shifts the water chemistry toward a higher pH level. Additionally, when water evaporates, it leaves all the hardness minerals in the aquarium. Some aquarists use distilled water to replace evaporated water. (Other kinds of bottled water like spring water are usually high in minerals and will increase water hardness.)The best way to manage water hardness levels is the Tap Water Filter. The Tap Water Filter will remove all minerals and pollutants from tap water, permitting easy adjustment of pH and water hardness.

The pH of some synthetic marine salts stabilize below or above the recommended 8.2 to 8.4 pH range. Even though the KH is acceptable, the pH may not be right for the aquarium.

Water with very low KH may experience wide fluctuations in pH because the buffering capacity is low and small changes make big swings. A high KH level indicates that the pH will be very stable. High KH is almost always associated with a high pH.

A new freshwater aquarium will not immediately have the right balance of bacteria, and ammonia will build up. It may surge to 6 ppm (mg/L) or more, and then fall rapidly as the biological filter becomes established. The ammonia will be converted to nitrite (also toxic), then to nitrate. This process may take several weeks. In an established aquarium, the ammonia level should always remain at 0 ppm (mg/L). However, as with any natural process, imbalances can occur.. Over feeding and keeping too many fish in the aquarium will also cause an ammonia problem.

Because salt remains behind when water evaporates from your tank, you will probably be using pure water to top off the tank. If you do it frequently, your specific gravity will not fluxuate wildly. However, if you wait too long, the salinity will be rising through evaporation and a large addition of water could drop the specific gravity too quickly for the health of the tank inhabitants.

It is a good idea to measure the specific gravity of your tank before and after each water change as well, taking care to match the new water to the old water in order not to "shock" the inhabitants of the tank with too large a deviation too quickly.

It is essential to maintain the correct salinity in a saltwater aquarium because of the fine balance between the concentration of salts in a fishes body tissue, compared with that in the surrounding water. If the salinity of the water is outside the range of naturally occurring salinities, then fish and invertebrates will experience osmotic stress as they try to adjust. Osmotic stress can lead to death.

The most problematic salinity variation is the gradual increase of Salinity due to evaporation. Constant and accurate measurements are essential.