Fishless cycle ammonia calculator
A fishless cycle feeds a new filter with ammonia until bacteria grow that can process it. Enter your tank and the ammonia you have, and the page works out how much to add for the target reading, with the arithmetic underneath.
The arithmetic
Water to dose for: 20 gal minus 20% = 16 gal (60.6 L).
Dr. Tim's product page gives 4 drops per gallon for 2 ppm, so the drops scale with the target: 16 gal × 4 drops × (2 ÷ 2) = 64 drops. At 0.05 mL per drop (1 tsp = 5 mL = 100 drops in the guide), that is 3.2 mL.
What Dr. Tim's says
The Bulk Reef Supply product page (opened 2026-10-18) says to add 4 drops of the ammonium chloride solution per gallon "to achieve an ammonia concentration of 2 mg/L-N (2ppm)", and not to exceed 5 mg/L. It also says that since November 2016 the solution takes 4 drops per gallon where the old one took 1, so the label on your bottle wins.
The fishless cycling guide on the same site (opened 2026-10-18) adds the rest of the routine:
- Reduce the calculated water volume by about 20% for substrate and decorations. Its example: a 30 gallon tank is dosed as 24 gallons.
- Day 3: if ammonia or nitrite reads under 5 ppm, add another 4 drops per gallon. If over 5 ppm, add no more (and the guide says not to go above 5 ppm).
- After day 7 or 8, when ammonia and nitrite are zero or below 0.5 ppm, add 2 ppm again (4 drops per gallon). If the tank brings it below 0.2 ppm within 24 hours, the guide says it is safe to start adding fish.
- Conversions for large tanks: 1 tsp = 5 mL = 100 drops.
Using household ammonia
Household ammonia is a common substitute. A FishProfiles forum thread (opened 2026-10-18) quotes a guide saying household cleaner grades run 4 to 10%, and that a bottle left open for a long time gets weaker. One poster worked out about 5.7 mL for 5 ppm in a 30 gallon tank with a 10% bottle. Another got there by adding a syringe at a time and testing: 8 to 9 mL.
Those are two readers' reports. This page uses the same method as the first poster, plus the density of the solution and the choice of nitrogen or NH3. For 30 gal, 5 ppm as NH3 and no displacement it gives about 5.9 mL, close to the poster's 5.7 mL. With the default nitrogen basis the same case comes to about 7.2 mL.
Use plain ammonia. A fishless cycling guide on FishProfiles (opened 2026-10-18) says it should be free of surfactants, perfumes and colorants. It suggests reading the ingredients, and shaking the bottle: ammonia with surfactants foams, and good ammonia does not.
Because bottle strength, drop size and test kits vary, my suggestion is to dose a little less than the result, test, and top up toward your target.
Related pages
Not sure how much water is in the tank? The tank volume calculator subtracts gravel and rock. Overshot the target? The ammonia and nitrite page shows what water changes do to a reading, and the dechlorinator calculator covers the conditioner for the new water. The FishProfiles guide above advises against conditioners that also bind ammonia during a cycle, so check what yours does.