Spring Start-Up of a Garden Pond

How to bring a garden pond back into service after winter: restarting the biofilter, first ammonia and nitrite tests, temperature-led feeding, the spring disease window and plant care.

Why spring is a critical period

Water warms slowly in spring, pathogens resume activity before the fish do, and the immune system of the fish recovers last. Practical Fishkeeping notes that the antibody response of koi carp is suppressed while water stays below 14 °C, and the University of Florida (UF/IFAS) fact sheet on stress in fish reports that cold water halts the activity of immune "killer cells" and impairs the release of antibodies. Wikipedia describes koi immune systems as very weak below 10 °C. The result is a window in early spring in which pathogens are already active but the fish cannot yet mount a full defence.

Restarting filtration

A koi pond normally relies on a pump and a filtration unit to keep the water clear (Wikipedia). If the filter was switched off or drained for winter, its population of nitrifying bacteria has to rebuild. UF/IFAS states that a biofilter needs three to eight weeks to become established at 25–27 °C and longer at cooler temperatures; a second UF/IFAS publication gives six to eight weeks for a new biofilter. A restarted pond filter is therefore effectively immature for several weeks in cold spring water.

Nitrifying bacteria also need oxygen and alkalinity. UF/IFAS gives a minimum total alkalinity above 20 mg/L for the process to run at all, recommends 100–180 mg/L for good biofiltration, and places the preferred pH of nitrifiers at 7.2–8.2 (7–9 acceptable). Parasiticides and antibacterial agents can suppress nitrification, so medicating a pond whose filter has just been restarted slows the recovery of the biofilter.

Rinse biological media only in water taken from the pond; chlorine and chloramine in tap water kill the beneficial bacteria (Practical Fishkeeping).
When media must be replaced, change no more than one third at a time and wait about a week before replacing the next third (Practical Fishkeeping).
Aim for a turnover of the full pond volume at least once every two hours; for a 1,000-litre pond this means a flow of at least 500 L/h (Practical Fishkeeping).
Keep dissolved oxygen above 5 mg/L at all times; UF/IFAS names this as the first line of disease prevention.

First water tests

Ammonia is the first parameter to check once the filter is running and the fish start eating. According to UF/IFAS, un-ionised ammonia (NH3) is about 100 times more toxic to fish than the ionised form; tissue damage begins above 0.05 mg/L of un-ionised ammonia and fish typically die at 2.0 mg/L. The share of ammonia present as NH3 rises with pH and with temperature, so the same total reading becomes more dangerous as the pond warms. Penn State Extension considers any measurable ammonia-nitrogen above 0.1 mg/L detrimental to pond health. UF/IFAS gives 0.10 mg/L as the threshold at which nitrite becomes toxic.

Test kits report total ammonia nitrogen (TAN). UF/IFAS notes that Nessler-type kits are disturbed by formalin and ammonia-binding conditioners, while salicylate kits are not. UF/IFAS recommends weekly testing, an immediate test whenever fish behaviour changes, and alkalinity and pH checks weekly to monthly.

If ammonia or nitrite is detected: reduce or stop feeding for one to three days (UF/IFAS).
Carry out partial water changes of 25–50 % in small systems (UF/IFAS).
Maintain aeration so that dissolved oxygen stays above 5 mg/L (UF/IFAS).
Re-test daily until both readings return to zero, then resume feeding gradually.

Gradual feeding

Koi digestion slows almost to a standstill in cold water, and Wikipedia reports that feeding is not recommended below 10 °C; appetite returns only as the water warms in spring. Practical Fishkeeping describes a stepwise return to normal diet driven by water temperature, not air temperature: below 10 °C a wheat-germ-based food only; between 10 and 15 °C a mixture of wheat-germ food and a higher-protein food; from 16 °C upwards a high-protein diet. Fish metabolism peaks in the warm months, when protein demand is highest.

The spring disease window

Bacterial ulcer disease is the classic spring problem of koi and goldfish. The Merck Veterinary Manual describes Aeromonas salmonicida, a gram-negative rod, as a cause of disease in goldfish and koi: acute cases show haemorrhages on fins, tail, muscle and gills, while chronic cases develop focal swellings that break down into deep, crater-like ulcers. Outbreaks are linked to poor water quality, organic loading, handling and transport, marked temperature changes and low oxygen. A peer-reviewed carp study (PubMed Central) describes the related Aeromonas hydrophila as a widespread opportunistic pathogen causing haemorrhages, inflammation and ulcers.

Parasites also reactivate with temperature. Practical Fishkeeping notes that the white-spot parasite completes its life cycle every six to seven days at 20 °C but may take months below 10 °C, and lists flicking, flashing, excess mucus, lethargy, laboured breathing and ragged fins as signs of infection. Wikipedia reports no known treatment for koi herpesvirus (KHV) or spring viraemia of carp (SVC), which is why new koi are quarantined for several weeks.

Watch fish daily as soon as they start moving: isolated individuals, clamped fins, reddened areas or ulcers are early signs.
Move affected fish to a quarantine container. Practical Fishkeeping cites 3 g of salt per litre in quarantine to reduce osmotic stress and stimulate mucus production; do not add salt to a pond with salt-sensitive plants.
Broad-spectrum antiparasitic treatments should be used only when a parasite has been confirmed (Practical Fishkeeping).
Bacterial ulcers require appropriate antimicrobial therapy chosen by a veterinarian (Merck Veterinary Manual).

Plant trimming and surface cover

Dead marginal vegetation left standing from autumn should be cut back to a few centimetres above the soil line before spring growth begins, and rhizomatous plants can be divided at the same time (Clemson Cooperative Extension). Practical Fishkeeping advises keeping about two-thirds of the pond surface free of plants, because aquatic plants release oxygen by day but consume it at night. Penn State Extension adds that a surface completely covered by floating plants cuts off light and prevents gas exchange between water and air, leaving the pond oxygen-poor.