Flow and Water Change Requirements for Nursery Ponds

Like all fish rearing ponds, nursery ponds require a steady supply of conditioned fresh water and routine water changes. However, the small size and delicate bodies of fish fry and fingerlings makes it tricky to accomplish these goals without damaging your stock. Setting up a pond designed to take advantage of an existing water supply can solve the flow issue and provide enough water to replace what you remove for a water change. With a trickle setup, you can even eliminate the need for a manual water change by having a constant flow in and out of the nursery pond.

Incoming Water Supply

Setting up a lined raceway-style nursery pond is often the best choice for cold water fish like trout and fish that need constantly water turnover. Nursery ponds are particularly suited to this kind of flow-through design because fry in particular need constant water changes for rapid growth and high water quality. However, don’t assume that an existing river or other water supply has the water quality you need for sensitive fry or challenging species. Test any water you’re interested in using before inquiring about permits and estimating fees for annual water use. Don’t forget to keep up with routine tests to ensure there’s no runoff or contaminants affecting your water supply.

Treating Tap Water

Water from a river or lake diverted for a raceway nursery pond may have too much nitrogen or chemical residues from nearby roadways or manufacturing plants. However, tap water supplied for drinking water isn’t automatically a good choice for filling static nursery ponds. Most water supplies, even for human consumption, are high in chlorine and related compounds that can be seriously damaging to the gills of tiny fry. Tap water usually needs testing and treatment before each water change or addition to compensate for evaporation. Set aside 24 to 48 hours for proper treatment since chlorine largely evaporates to leave behind water suitable for fish ponds.

Water Changes vs Replacement

With the relatively shallow depth and large surface area required for a nursery pond, many designs experience such a high evaporation rate that up to 10% of the total volume is lost each week. If you’re losing that much and refilling it with a steady supply of fresh water, there’s no need for a separate water change. Estimate how much your pond will lose according to local conditions that affect evaporation, such as average wind speed, sun exposure, and temperatures. Add that to the 10% total you’re aiming to change each week to determine what’s still necessary. For example, a pond experiencing a steady 4% evaporation rate week after week only needs a 6% water change to keep the fry healthy and growing.

Filtration Can’t Prevent Water Changes

There is a common misconception that mechanical and biological filters replace or eliminate the need for water changes. However, filters are only designed to keep water quality high between changes, not eliminate them entirely. Without any filtration, you’ll need to change out 10% or more of your total water volume per day rather than per week to keep the fish happy. With extensive filtration and sludge removal after each batch of fry, you may be able to stretch your water changes to twice a month or even once a month as long as you observe relatively low stocking rates.

Building a Pond Around a Natural Water Supply

Don’t assume that an existing water supply is available for diverting into a fish hatchery or rearing operation. Many states restrict new facilities to only those serving public fish stocking program, while others charge such high permitting and planning fees that it’s financial unfeasible to build your ponds around them. Static ponds you fill from a well or public water supply can work just as well as flow-through raceways, even if you need extra aeration or turbulence for species that like moving water.

Regardless of whether you choose static or flow-through nursery pond designs, durable and flexible liners from BTL Liners are an essential method of controlling water loss. Since flow-through raceways rely on the water returning to the original waterway, liners are the best way to return as much water as possible to keep permitting costs under control. Static ponds loss water at a fraction of the usual rate when lined, reducing the water you need to stay on top of evaporation and routine changes.

Liners by BTL

AquaArmor Pond Liner

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