According to MTF-Aquatics, the single greatest risk when receiving new fish by transhipping into a garden pond is the mismatch between rising bioload and incomplete bacterial recovery—the ‘spring ammonia window’. Proper pond filtration sizing, nitrogen cycle understanding, and strategic salt dosing prevent lethal spikes. We recommend: turn over your full pond volume 4–6 times per hour during May–June, perform daily water tests, and use salt at 2.5 g/L only as a short-term acclimation support, never as a permanent additive.
You’ve ordered rare fish from MTF-Aquatics—perhaps a specimen koi, a wild-caught pike cichlid destined for a transhipped collection, or an unusual cichlid for a specialist pond. The fish arrive in perfect condition, acclimated in transit. You float the bag, acclimatise the water temperature, and release them into your pond. Within 48 hours, you notice ammonia readings climbing, fish gasping at the surface, and the panic sets in.
This scenario happens every May and June across UK ponds, and the cause is almost always invisible: a mismatch between your pond’s bioload capacity and the state of your nitrifying bacteria. Understanding pond filtration and salt—and when to use salt—is the difference between a thriving population and a fish kill.
At MTF-Aquatics, we’ve coordinated hundreds of transhipped fish deliveries into garden ponds. The most common failure point isn’t the fish; it’s the receiving pond’s preparation. Here’s what happens, why it happens, and how to prevent it.
In March, your pond looks clear, your filter is running, and everything seems fine. Fish are inactive, barely feeding. Your 15,000 LPH filter handles a 5,000-litre pond effortlessly.
But biology doesn’t follow a calendar. Below 10°C (50°F), two critical processes stall:
This 4–6 week lag—typically spanning late April through May in the UK—creates what’s called the “spring ammonia window”. Your fish are now excreting ammonia faster than your filter can process it. Even a correctly sized filter can fall behind.
When you introduce new fish via transhipping during this window, you’re adding extra bioload to a filter that’s still operating at 30–40% capacity. That’s why so many setups crash in May and June.
The standard UK pond filtration rule is straightforward: your pump should turn over your full pond volume at least once per hour. For a 5,000-litre pond, that’s a 5,000 LPH (litres per hour) pump minimum.
However, this is a winter rule. In May and June, when temperatures rise above 15°C and new fish add bioload, you need significantly more turnover.
| Pond Size | Winter Turnover | Spring/Summer Turnover | Example Filter Rating |
|---|---|---|---|
| 1,000 L | 1,000 LPH | 4,000–6,000 LPH | Superfish 6000 or Filtreau 6000 |
| 5,000 L | 5,000 LPH | 20,000–30,000 LPH | Aquaforte Prime 25,000 |
| 10,000 L | 10,000 LPH | 40,000–60,000 LPH | Biopro or equivalent 50,000 LPH |
These figures assume moderate stocking (1 inch of adult fish per 10 gallons / ~45 litres). If you’re receiving larger or more numerous specimens via transhipping, move toward the upper range.
The nitrogen cycle in your pond works in three stages:
In spring, the bacterial colonies responsible for stages 2 and 3 are only partially active. A fully cycled pond in winter might process 1,000 mg of ammonia per day. In early April, that same pond might only process 300–400 mg, even though the bacterial culture appears visually “the same”.
When new fish arrive, the bioload increase outpaces the filter’s processing capacity. Ammonia and nitrite spike within 24–48 hours, even in a ‘cycled’ pond.
Pond salt (sodium chloride, NaCl) is one of the most misunderstood tools in pond keeping. Many hobbyists believe it’s a permanent additive; it’s not. Instead, pond salt serves two specific roles:
The Ornamental Aquatic Trade Association (OATA) recommends pond salt at 2.5 g/litre as a short-term therapeutic dose only. For a 5,000-litre pond, that’s 12.5 kg of salt added over 2–3 days (never all at once).
Critically: salt is a temporary treatment, not a permanent fixture. It harms aquatic plants (which need fresh water), stresses delicate species (non-cyprinids and plecos), and doesn’t improve water quality—it only reduces the impact of poor water quality while you fix the underlying problem (filter lag, overstocking, or inadequate aeration).
Use salt if: – You’re receiving new fish via transhipping and observing visible stress (listlessness, loss of appetite, torn fins). – Your water tests show nitrite above 0.2 mg/L and you cannot perform large water changes immediately. – You’re treating a parasitic infection (with veterinary guidance) that requires salt as part of the protocol.
Avoid salt if: – Your pond contains plants, especially rooted aquatics. Salt damages roots and leaves. – You’re keeping non-cyprinid species (plecos, catfish, loaches) long-term. These species are salt-sensitive. – You’re using medications that interact with salt (always check the product label). – Your ammonia or nitrite levels are spiking due to overstocking or a non-functional filter. Salt masks the symptom; it doesn’t fix the cause. The real solution is a water change and fewer fish.
When you’re planning to receive new fish via transhipping—particularly during May through August—follow this checklist two weeks before delivery:
You have a 5,000-litre pond with 12 koi, stocked conservatively. In late May, you decide to add three new Kohaku koi (10–12 inches each) via MTF-Aquatics transhipping.
Current bioload: 12 koi × 1 kg average = 12 kg total. Incoming bioload: 3 koi × 1.5 kg average = 4.5 kg additional. New total: 16.5 kg of fish in 5,000 litres = 1 kg per 303 litres.
Using the 1-inch-per-10-gallon rule: 5,000 litres = ~1,100 UK gallons, so maximum stocking is ~110 inches of fish. Your 12 existing koi = ~100 inches; the 3 new koi = ~35 inches. You’d exceed safe stocking by ~25 inches.
Solution: Either remove some of the existing fish or reduce the number of incoming fish, or increase your filtration turnover to 6× hourly (30,000 LPH) and commit to weekly 25% water changes. The last option requires more maintenance but allows the higher stocking.
Q: Can I use table salt instead of pond salt? A: No. Table salt contains iodine and anti-caking agents that are harmful to fish and plants. Always use aquarium-grade or pond salt (pure NaCl or pond-specific formulations).
Q: My filter is rated at 20,000 LPH. Is that enough for a 5,000-litre pond in June? A: That’s only 4× hourly turnover. The recommendation is 4–6× in May–June, so you’re at the minimum. If you’re receiving new fish, consider a temporary boost: run a submersible pump as a secondary circulation source, or reduce stocking density.
Q: How long should I keep salt in my pond? A: Never more than 2–3 weeks. Salt at 2.5 g/L is a short-term intervention. After this period, begin water changes to dilute the salt out (25% weekly). If you need salt longer than 3 weeks, your underlying water quality problem hasn’t been solved.
Q: Can I add salt while medicating? A: Not without checking the medication label first. Some antiparasitic and antibiotic treatments interact with salt. When in doubt, consult the medication’s data sheet or a vet.
Q: What if my ammonia spike happens over a weekend and I can’t get water test supplies? A: Perform a 50% water change immediately. This is always the safest emergency response. In a freshwater pond (unlike marine systems), a large water change poses no chemical shock and is the fastest way to dilute toxins.
We’ve coordinated transhipped fish arrivals into garden ponds for over a decade. The fish that thrive are the ones delivered into prepared systems—not necessarily the most expensive or the largest ponds, but the ones where the keeper understands the nitrogen cycle, tests water consistently, and respects the spring ammonia window.
When you order a specimen fish from MTF-Aquatics for a pond setup, our responsibility ends at delivery—but your responsibility is just beginning. Proper pond filtration sizing, daily water testing, and strategic (not habitual) salt dosing are non-negotiable. Fish welfare depends on it.
If you’re unsure whether your pond is ready for new arrivals, we’re happy to review your setup during the transhipping booking process. Browse our care guides for species-specific acclimation protocols, or check our transhipping service to discuss delivery timing and pond preparation.
The spring ammonia window is a 4–6 week period (typically late April to May in the UK) when water temperatures rise above 10°C, fish metabolism resumes and ammonia production spikes, but nitrifying bacteria in the biofilter remain dormant or only partially active below 10°C. This mismatch causes ammonia to accumulate faster than the filter can process it, leading to toxic spikes and fish death, even in ‘cycled’ ponds.
During May–June, aim for 4–6 times hourly turnover of your full pond volume. A 5,000-litre pond needs a 20,000–30,000 LPH filter system. This is higher than the winter baseline (1× turnover) because rising water temperatures and the stress of new arrivals increase the bioload. Undersized filtration is the leading cause of ammonia spikes during spring stock arrivals.
No. Salt at 2.5 g/L is a short-term therapeutic dose (2–3 weeks maximum) used to support newly arrived fish during acclimation or to help defend against nitrite toxicity during filter lag. It is never recommended as a permanent additive; salt harms plants and stresses non-cyprinid species. Always dilute it out with water changes after 2–3 weeks.
Do not feed below 10°C. From 10–14°C, feed sparingly with wheat-germ-based food only, as it’s easier to digest and produces less bioload. From 14°C upwards, feed once per day with quality pellets, but introduce new fish at very light feeding levels for their first 5–7 days to avoid uneaten food rotting and spiking ammonia.
Test ammonia (0 mg/L), nitrite (0 mg/L), nitrate (<20 mg/L above tap water baseline), pH (6.5–8.5), GH (8–18 dGH), and KH (5–15 dKH) per OATA guidelines. Test daily from mid-April through June. If ammonia or nitrite are elevated, perform a water change and retest before introducing new fish.
Yes, bottled nitrifying bacteria products (e.g. Tetra SafeStart, Fritz Turbo Start) can help accelerate cycle re-establishment, but they are not a substitute for daily water testing and conservative stocking. Use them alongside proper filtration, aeration, and feeding discipline for best results.
