Natural Pond Handbook

26 · Top-Up Water & Rainwater

Evaporation and splashing mean you will add water. What you add matters as much as what's already there — tap water can shock a pond if used carelessly.

Tap water: chlorine and chloramine

  • Chlorine off-gasses if you leave water standing 24–48 hours, but many utilities now use chloramine, which does not dissipate and is toxic to bacteria and fish.
  • Always dechlorinate tap top-ups with a pond-safe conditioner that neutralizes both chlorine and chloramine, or use a hose-end dechlorinator on auto-top-up lines.

Softened and RO water

  • Water-softener outflow is salty — never use it; the sodium harms fish, plants, and amphibians.
  • Reverse-osmosis (RO) water is pure but lacks buffers; useful for top-up in small amounts but will lower KH if used exclusively — remineralize if needed.

Rainwater harvesting

Rain is "free" and chlorine-free, but note:

  • Roof runoff can be slightly acidic and low in KH/buffers — large additions may drop pH.
  • Avoid first-flush from roofs coated in lichen, bird droppings, or treated timber.
  • Use a covered butt/tank to prevent mosquito breeding and debris.

How much will you need?

top_up_needed ≈ evaporation + splash − rain Hot, windy, fountained ponds lose the most.

Best practice

  • Top up little and often rather than one huge dump.
  • Match temperature roughly — very cold top-ups shock fish in summer.
  • An auto-top-up valve + inline dechlorinator is the lowest-maintenance solution.
Avoid Never top up directly from a domestic water softener. The added sodium will quietly poison the ecosystem.
Automatic top-up setup
  1. Install a float valve in a hidden compartment (box, rockery, or skimmer box).
  2. Connect to a garden hose or mains supply via a pressure-reducing valve.
  3. Set the float to activate 50 mm below the normal water level.
  4. Test by lowering the water manually and confirming the valve opens.
  5. Insulate exposed pipework in cold climates.
Chlorine and chloramine Tap water contains chlorine or chloramine, both toxic to fish and beneficial bacteria. Always add dechlorinator when topping up, or fit a carbon filter on the supply line.