Guideline – Freshwater:
A good principle is one inch of fish per gallon of water but there can be exceptions to this norm. The principle of an inch of fish per gallon of water is dependent on a common square aquarium of normal height. If you have, say a round-shaped aquarium, or a very tall aquarium, put one inch of fish for every 12 square inches of aquarium surface area. Consider some extra space for them to allow for fish growing.
For brand new home aquariums, start with 30% of the total volume of the tank in inches. Hold on a minimum of two weeks before adding fish. 3 weeks should be perfect. Do a 25% water swap just before you add new fish, not after it. Add in batches of 100% of the number of inches of fish the tank actually holds, until the aquarium is filled to 90% of the total volume. Then let the development of the fish you own to occupy the aquarium tank to its potential.
Guidelines – Saltwater
Saltwater possesses much less mixed oxygen than freshwater. The principle we use for saltwater is three inches of fish for every square foot of surface area. Your live rock must be fully cured before you can add any fish or invertebrates to your aquarium. And try not to neglect the curing process, which triggers the nitrogen cycle, usually takes 4-5 weeks. Through this time, you must also carry out weekly 25% water changes.
For new aquariums, start with 25% of the total volume of the tank in inches. Hold on at five or six weeks and analyze the water for ammonia and nitrite. If you do not get a zero reading for either tests, carry out another 25% water change and wait another week. Keep going with the procedure until both tests show zero. Then add not more than 50% of the inches of fish you currently have in the tank (if you currently have four inches of fish, then add two inches more). Do a 25% water replacement prior to adding new fish, NOT right away. And keep in mind to not to add fish more often than every two or three weeks. Fill the tank to 90% of the total volume. Then let the progress of the fish you have to fill the home aquarium to its full potential.