You walk outside, look at your pool, and the water is green. Maybe it is light green and hazy, maybe it is dark green and you cannot see the bottom. Either way, you have an algae bloom, and it is one of the most common pool problems homeowners deal with in Tennessee — especially during the long, humid summers around Dickson County.
The good news is that green pool water is fixable. The bad news is that it does not fix itself, and the longer you wait, the harder it gets. Here is exactly what causes it and how to get your pool water clear again.
What Causes Green Pool Water?
Green water is almost always algae. Algae are microscopic organisms that thrive in warm, stagnant water with low sanitizer levels. In Middle Tennessee, the conditions are ideal for algae growth from late April through October — water temperatures above 80°F, intense sunlight, high humidity, and afternoon thunderstorms that dump organic debris and dilute your pool chemicals.
Here are the most common reasons your pool turned green:
- Low free chlorine — When free chlorine drops below 1 ppm, algae spores already in the water start multiplying rapidly. A single hot day with heavy bather load can burn through your chlorine faster than you expect.
- Poor circulation — Dead spots in the pool where water does not move are where algae takes hold first. Clogged skimmer baskets, dirty pump strainer baskets, or a pool pump not running long enough create stagnant zones.
- Dirty or clogged pool filter — Whether you have a sand filter, cartridge filter, or DE filter (diatomaceous earth), a dirty filter cannot trap algae cells and particles effectively. High filter pressure is a sign your filter media needs cleaning or replacement.
- Unbalanced water chemistry — High pH reduces chlorine effectiveness dramatically. At a pH of 8.0, your chlorine is less than half as effective as it is at 7.2. High cyanuric acid (stabilizer) levels above 80 ppm also lock up free chlorine.
- Rain and storms — Heavy rain dilutes sanitizer, introduces phosphates and nitrates from runoff, and washes leaves, pollen, and dirt into the pool. After a Tennessee thunderstorm, your pool chemistry can shift overnight.
- Not brushing the pool walls and floor — Algae clings to pool surfaces, especially rough plaster, pebble finishes, grout lines, and around pool steps and ladders. If you only skim and add chlorine but never brush, algae builds a biofilm that resists sanitizer.
Types of Pool Algae
Not all algae looks the same, and knowing which type you are dealing with helps you treat it effectively.
- Green algae — The most common type. It floats freely in the water, making it look cloudy green, or clings to walls and the pool floor as a slimy green film. Green algae responds well to shocking and brushing.
- Yellow algae (mustard algae) — Shows up as a yellowish, sandy-looking film on shady pool walls, steps, and the shallow end floor. Mustard algae is chlorine-resistant and harder to kill than green algae. It requires higher shock doses and thorough brushing.
- Black algae — The toughest to get rid of. Black algae embeds deep into plaster, concrete, and gunite pool surfaces with root-like structures. It appears as dark blue-green or black spots, often on the pool floor or walls. Aggressive brushing with a stainless steel algae brush (on plaster pools) and repeated heavy shocking is necessary.
Step-by-Step: How to Clear a Green Pool
Whether you have a light green tint or full swamp water, the process is the same — you just need more of everything the worse it is.
1. Test Your Water Chemistry
Before you add anything, test the water. Use a liquid test kit or test strips to check free chlorine, pH, total alkalinity, cyanuric acid (CYA), and calcium hardness. You need to know where you are starting so you do not waste chemicals.
Pay special attention to cyanuric acid. If your CYA is above 80–100 ppm, your chlorine is essentially locked up and cannot kill algae no matter how much you add. The only fix for high stabilizer is a partial drain and refill with fresh water.
2. Clean Out Debris
Use a leaf net or skimmer net to remove as much floating and sunken debris as you can — leaves, sticks, bugs, anything organic. This material consumes chlorine and feeds algae. Empty your skimmer basket, pump strainer basket, and clean the pool cleaner bag if you use an automatic pool cleaner or robotic pool cleaner.
3. Brush Everything
Brush the pool walls, pool floor, steps, behind the pool ladder, around the main drain cover, and inside the skimmer throat. Use a nylon pool brush on vinyl liner pools and fiberglass pools, or a stainless steel brush on plaster, gunite, and concrete pools. Brushing breaks up the algae biofilm and exposes it to the chlorine you are about to add.
Do not skip this step. Pool shock alone does not remove algae that is clinging to surfaces. You need mechanical scrubbing to break the protective layer so the sanitizer can penetrate and kill it.
4. Shock the Pool
Pool shock is a large dose of chlorine (or non-chlorine oxidizer) that raises the free chlorine level high enough to reach breakpoint chlorination — the point where chlorine overwhelms and destroys all the organic contaminants, chloramines, and algae in the water.
For a green pool, you need to triple-shock or quadruple-shock depending on severity. A general guide:
- Light green, can still see the bottom — 2 lbs of calcium hypochlorite shock per 10,000 gallons
- Medium green, bottom barely visible — 3 lbs per 10,000 gallons
- Dark green, cannot see the bottom — 4 lbs per 10,000 gallons
Dissolve granular shock in a bucket of pool water before adding it. Broadcast liquid chlorine (sodium hypochlorite) directly. Shock at dusk or night — UV from sunlight breaks down unstabilized chlorine within hours. Run the pool pump and filter system continuously for at least 24 hours after shocking.
5. Run the Filter and Backwash Repeatedly
Your pool filter is doing the heavy lifting now. As dead algae turns grey and clouds the water, the filter traps it. Monitor the filter pressure gauge — when pressure rises 8–10 psi above the clean starting pressure, it is time to backwash (sand filter or DE filter) or rinse the cartridge (cartridge filter).
You may need to backwash multiple times in the first 24–48 hours. With a severe bloom, vacuuming to waste bypasses the filter entirely and sends the dead algae water straight out of the pool through the waste line. This drops the water level, so keep a hose running to replace it.
If you have a sand filter, consider adding DE powder through the skimmer after backwashing. A thin coat of diatomaceous earth on the sand bed helps catch finer algae particles that sand alone misses.
6. Retest and Balance
After 24 hours of circulation, test again. Free chlorine should still be elevated — if it has dropped back to zero, the algae consumed it all and you need to shock again. Keep shocking and filtering until the free chlorine level holds above 1 ppm and the water is changing from green to cloudy blue or white.
Once the green is gone, balance your pool water chemistry:
- pH — 7.2 to 7.6 (ideal is 7.4)
- Total alkalinity — 80 to 120 ppm
- Free chlorine — 2 to 4 ppm for daily maintenance
- Cyanuric acid (stabilizer) — 30 to 50 ppm for chlorine pools, 60 to 80 ppm for saltwater pools
- Calcium hardness — 200 to 400 ppm
Adjust total alkalinity first with sodium bicarbonate (baking soda) to raise it or muriatic acid to lower it. Then adjust pH with soda ash (sodium carbonate) to raise or muriatic acid to lower. Getting alkalinity and pH right makes everything else easier.
7. Vacuum and Final Cleanup
Dead algae settles on the pool floor as a fine grey or white powder. Vacuum it slowly — stirring it up just makes the filter work harder. Use the waste setting on your multiport valve if you have a lot of sediment, or vacuum through the filter and backwash immediately after.
Run a pool clarifier or flocculant if the water is still hazy after vacuuming. Clarifier clumps tiny particles together so the filter can catch them. Flocculant drops everything to the floor for you to vacuum to waste — faster results but uses more water.
How to Prevent Algae From Coming Back
Clearing a green pool is a lot of work. Prevention is always easier:
- Maintain free chlorine between 2–4 ppm — Test at least twice a week during swim season. Use a reliable liquid DPD test kit or FAS-DPD drop test for accuracy. Pool test strips work in a pinch but are less precise.
- Run the pool pump long enough — Your pool water should turn over at least once per day. For most residential pools in the 15,000 to 25,000 gallon range, that means 8 to 12 hours of pump run time. Variable speed pool pumps let you run longer at lower RPM for better filtration and lower energy bills.
- Brush weekly — Brush the walls, floor, steps, and behind ladders once a week. This prevents algae from establishing a biofilm even when chlorine is in range.
- Clean the filter regularly — Backwash sand and DE filters when pressure rises 8–10 psi. Clean cartridge filter elements every 4–6 weeks during heavy use. Replace filter sand every 5–7 years, DE grids every 7–10 years, and cartridges every 1–2 years.
- Keep cyanuric acid in check — CYA protects chlorine from UV breakdown, but too much reduces its killing power. Drain and refill partially if CYA creeps above 80 ppm.
- Shock after heavy use or storms — A pool party with a dozen swimmers, a heavy rain, or a pollen dump all warrant an extra shock dose to reset chlorine demand.
- Use an algaecide as a preventive — A weekly maintenance dose of copper-based or polyquat algaecide gives you a backup layer of protection. It is not a substitute for proper chlorine levels, but it helps when chemistry dips temporarily.
- Trim trees and landscaping — Less organic debris falling in means less chlorine demand and fewer nutrients for algae. Keep branches trimmed back from the pool area.
Saltwater Pools Get Algae Too
If you have a saltwater pool with a salt chlorine generator (salt cell), you are not immune to algae. Your salt system makes chlorine, but it can only produce so much per day. During heat waves, high bather loads, or after storms, chlorine demand can exceed what the salt cell generates.
Check your salt level (2700–3400 ppm for most systems), inspect the salt cell for calcium scale buildup, and be ready to supplement with granular shock or liquid chlorine when needed. Clean the salt cell with a mild acid wash every few months to keep it generating efficiently.
When to Call a Professional
If your pool has been green for weeks, if you are dealing with black algae in a plaster or gunite pool, if the filter pressure is maxed out and backwashing is not helping, or if you have drained and shocked multiple times with no improvement — it might be time to bring in help.
At Dickson Pool Cleaning Service, we deal with green pools, algae blooms, and water chemistry problems regularly across Dickson County. We serve Dickson, Charlotte, White Bluff, Burns, Vanleer, Slayden, and the surrounding areas. A one-time cleaning is $100, and our ongoing weekly and biweekly maintenance plans start at $65 per visit — check our plans page for details.