When a hot tub is properly balanced with the right chemicals, algae growth is not normally a problem you encounter. But if you unwittingly give algae an inch, it almost always takes a mile. It can be a huge pain to eradicate and is sometimes tough to keep it gone.
If you’ve already got an algae problem, however, there are actions you can take to stop it in its tracks and return to crystal clear water.
Your first goal is to figure out what type of algae you’re dealing with. Typical treatment involves brushing down the sides and floor of your hot tub, and then treating the water with spa chemicals like algaecide or pH modifier, but of course, the specifics vary depending on the algae species. There are a few different strains that can form in a hot tub environment, and all have varying degrees of hardiness.
Often brownish gray, this scum could be caused by a mineral like iron reacting in your water, or it could mean that the pH level is too high. Oftentimes treating the water with a pH decreaser will resolve the issue. If this type of algae is a recurring problem, try to maintain your pH at the lower end of 7.2-7.4 for a few months.
This is a sign of metal in your water. Dissolved metals such as copper or magnesium react with bromine and result in a greenish residue. It is the fastest growing type of scum, so if the algae bloom happens overnight, it is probably this type. An early warning sign of green algae is a sliminess to the bottom and sides of the hot tub. It is fairly easy to treat. If you suspect metals are the culprit, try using a pre-filter on your garden hose. Pre-filters remove 99% of contaminants and suspended solids from the water as it leaves the hose and enters your hot tub, so you don’t have to make up for it later while balancing your water.
A common type of scum that chlorine does not kill. This is the same type that causes blackish stains on bathroom tiles and in fish tanks. This usually indicates a filter problem. Remember that cartridge filters should generally be replaced every 1.5-2 years. Use a filter cleaner and spray with a garden hose to clear off a clogged or dirty filter, and make sure it dries totally before putting it back in place.
Usually features yellowish-green water and a yellowish-gold, powdery growth. This type of scum typically brushes away easily, but is highly resistant to treatment and thrives in shady conditions. Brush any algae from the surfaces, vacuum the waste, and then shock the water and treat it with an algaecide, following the dosage on the bottle. Brush down all surfaces again, and keep the pump on to allow the chemicals to circulate.
This type is also quite difficult to remove completely because the algae plants its roots deep. Unless those roots are killed completely, new algae will just regrow in the same spots. It is characterized by rough black patches on the walls and bottom and is most commonly caused by low chlorine levels and an inefficient filter. Treatment is similar to mustard algae, but you want to make sure you use a very strong algaecide and scrub the black spots thoroughly. Clean or backwash your filter afterward to get rid of any leftover spores.