NH₄
ammonia

• New, cycling tanks
• Heavily stocked and/or heavily fed aquariums
• Old, neglected tanks
• Use of antibiotics
• Aeration failure

• For established tanks:
  frequent water changes, removal of excess waste
• In emergencies: ammonia-removing or ammonia-binding products to help protect the fish in the short run

NO₂
nitrite

• Dissolve one tablespoon salt per 5 gallons water
• See "ammonia"

NO₃
nitrate

• Old, neglected tanks
• Source water

• Increase water change frequency and size (within reason)
• Check water source
• Reduce the load by removing fish
• Move up to a bigger aquarium

pH
acidity/alkalinity

• Freshwater: 6.5 to 7.5

• Low pH: older tanks, infrequent water changes

• Low pH: make small, frequent water changes, check the water source, add commercial buffers

• Saltwater: 7.8 to 8.5

• High pH: source water

• High pH: find a better water source, use commercial buffers

KH
carbonate hardness

• Freshwater: 4 to 6 dKH

• Pretty much the same

• Low KH: acids produced by waste

• Low KH: freshwater (and saltwater in some situations): change water frequently, add commercial buffers.

• Saltwater: 6 to 10 dKH

• Pretty much the same

• High KH: source water

• High KH: find another source for some or all of the tank water. Reverse osmosis water, which has a KH of near 0, can be mixed with source water to drop the KH proportionally in other words, using half R/O will drop the KH by half.

GH
general hardness

• Freshwater: possibly variable by species of fish

• Freshwater: 5 to 15 dKH

• GH too low: source water

• GH too low: add commercial buffers, change water source

• Saltwater: N/A

• Saltwater: off the chart

• GH too high: source water or calcareous decorations

• GH too high: use a different source for some or all of the water, remove calcareous decorations