By The Zagosa Team. Published .
Reviewed or updated .
You can do both on the same day, but only in the right order and with a wait between. Shock first, run the jets for at least an hour, then add algaecide. Flip the order, or stack them inside the same hour, and you waste both products and risk foaming.
Why order is everything
Most algaecides are quaternary ammonium compounds (quats). Chlorine goes after quats just as keenly as it goes after algae cell walls. If both sit at full strength at once, the chlorine devours the algaecide first and the algae get a free ride.
Shock goes in first because it is the bigger oxidising hit. Once it has dropped back into the bathing band over an hour or two, the algaecide can get to work without being wiped out immediately.
When you genuinely need algaecide
Hot tubs almost never do. The heat and the constant sanitiser presence keep algae a non-issue. If you are seeing green, the cause is nearly always too little sanitiser or biofilm rather than true algae.
Swim spas need it now and then, thanks to their larger volume and lower running temperature.
A garden tub left without sanitiser for weeks might need a one-off algaecide treatment alongside a heavy shock to clear the slime, but that is a rescue job, not routine.
The right routine if you do need both
Shock with chlorine to 10 ppm, jets on, cover off. Wait two hours.
Test free chlorine. Once it is below 5 ppm, dose algaecide as the bottle directs.
Leave the jets running another hour, then cover and leave overnight. Test again the next morning before anyone bathes.
Common questions
Will algaecide foam the tub up?
Quat-based algaecides will foam if dosed at the top of the bottle range. A little foam for a few hours is normal.
Is there a non-foaming algaecide?
Polyquat (polymeric quaternary ammonium) foams much less and is what most pros reach for in hot tubs when they need one at all.