Once a biofilter is initially acclimated, it contains the right types of bacteria. The next step is to ensure there are enough bacteria to handle the ammonia produced by the animals in the system. This is done by gradually increasing animal density in moderate steps, allowing time for the bacterial population to grow and meet the higher demand.
This phase of acclimation—adjusting to increased loading—is usually performed with the species being cultured. Because the system is already partially acclimated, peaks in total ammonia nitrogen (TAN) and nitrite are typically small and short-lived. As a general rule, an acclimated biofilter can adjust to a sudden increase in fish density or feeding level within about 72 hours.
Moderate step increases—less than one-third of the current load—usually occur without noticeable peaks. The height of any transitional peaks is primarily determined by the density of animals in the system, not by the size of the biofilter. For example, a system stocked at 0.25 pounds per gallon will exhibit nitrite peaks about half as high as a system stocked at 0.5 pounds per gallon. (See Table 3.5 for additional strategies to minimize transitional peaks.)
In practice, acclimation to increased loading happens naturally if bacteria and animals are allowed to grow together. Bacteria typically reproduce faster than the animals, maintaining the proper balance between biofilm and animal density. In a growout system, for example, once the biofilter is acclimated to fingerling density, the bacterial ecosystem will continue to maintain balance as the animals grow.
Management is most critical when natural growth processes are disrupted, such as by sudden, unnatural changes in stocking density or feed levels. During these times, careful monitoring and gradual adjustments are essential to maintain water quality and system stability.
| # | Procedure | How does it help? |
|---|---|---|
| 1 | Increase your waterloss from the system until the biofilters adjusts. | TAN and nitrite will be flushed with the water. |
| 2 | Discontinue or reduce feedrate during the transition. | TAN excretion rates from most animals increases with feeding. |
| 3 | Make loading increases in small increments (< 33 percent of current load) and separate steps by about 3 days. | Existing bacteria will absorb most of the increased load and reproduce rapidly. |
| 4 | Extend backwashing interval. | Decreases biofloc loss during the critical transition. |
| 5 | Adjust pH (7-8) and alkalinity (150-200 mg/L) to optimum range. | Accelerates reproduction of nitrifying bacteria. |
| 6 | Artificially increase the TAN loading prior to the increase by dosing of ammonia chloride (NH₄Cl) and sodium nitrate (NaNO₂) to a level of 1 mg-N/l. See calculations below. | Promotes growth of the critical nitrifying bacteria, enriching their density in the biofilm. |
Table 3.5 Things that Can be Done to Decrease Transitional Peaks of TAN and Nitrite When the Animal Density or Feed Rates are Increased