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Why Is My Pool Green?

Categories: Chemicals, Q&A Topics, Filters, Maintenance & Upkeep, Heaters
In my last post on How to Clean a Green Swimming Pool I addressed the steps to clearing up your pool after your pool turned green. But what you may not know is why your pool is turning green. It is important to know what is causing your water to turn green so you can prevent it in the future. This post will explain the most common reason pools turn green and how you might need to address specific issues to ensure your pool is green-free and safe for swimming.
The steps I outlined to clear up your green pool were:
  1. Check your water chemistry.
  2. Chemically treat your pool.
  3. Clean your filter. I've said it before, I'll say it again - A clean filter is paramount to clearing a green pool and keeping your pool clear. If your pump is the heart of your pool, the filter is its kidneys. Kidney failure is not a good thing!
Most of the time following these three steps will clear up a green pool that has been depleted of pool chemicals and has a dirty filter, sometimes you will need to repeat them a couple of times until the pool is completely clear. If those steps did not get your pool cleared up then you may have deeper issues. Let's discuss some of these common problems...
Problem #1 - an algae bloom that covers the walls of your pool.
This problem is fairly easy to clean up by following the three steps I outlined before. The basic method is to balance and super chlorinate your water while the pump runs constantly. This is followed by cleaning the filter. Follow my instructions here for a deep filter cleaning. This method may take several days and cleanings of the filter until you have clear water.
Spend some time brushing the pool walls and vacuum up the residual when it all settles to the floor. Be sure to clean your filter after visible signs of algae are gone.  
Treating your water with a product such as Coral Seas Yellow Out is a quick solution to clear green pools in 24 hours. An algaecide such as the Easy Care Algatec or United's Swamp Treat can also help. Using a phosphate remover such as Orend’s CV-700A to eliminate food for algae. Our blog post on Phosphates & Nitrates explains why a phosphate remover is so important. 
So, why did it happen? There are hundreds of forms of algae and they can be airborne as well as waterborne. Having the spores in your outdoor or indoor pool is an everyday occurrence and you really can’t stop them. Fertilizer, runoff from planters, leaves and organics, even bathing suits can bring in mold, mildew and algae spores to your pool. The key to keeping this from happening again is to be proactive by keeping your chemistry balanced with the addition of an algaecide and phosphate remover. This plus a clean filter should keep you green free.
Problem #2 - it’s been raining all night and the pool is green this morning.
This is a common problem that’s caused by a potential loss of chlorine and / or the accumulation of runoff into the pool that introduces phosphates, nitrates, or soil and organics that are robbing your chlorine.
To get your water cleaned up begin by emptying the skimmer and pump baskets. Fish out any debris floating on the surface of the water. A leaf rake like the Xcaliber LN4005 is better choice than a flat skimmer net for this job. It is always easier to remove heavy debris from a pool in the shallow end. As you dig debris out of the deep end, push whatever you cannot get in the net towards the shallow end, this is where a brush is handy. Note: If you have experienced a tornado, hurricane or sustained high winds, it is best to shut down the pump to prevent junk getting into the skimmer and pump baskets. After cleaning the floating stuff, you can turn on your pump to help you get your water cleared.
For this clean-up job I recommend having on hand a good brush with a metal handle. While you are at it, a good pool pole is worth its cost when you are fishing and brushing. Another tool that works great for removing leaves is a leaf eater. You attach it to your pool pole with a garden hose hooked up to the leaf eater, then you vacuum the leaves up. My recommendations are:
You will be agitating the water as you are cleaning up all the big stuff, so, you should be able to do two tasks at once, balance the chemicals and super chlorinate the pool. The chemical routine is the same as outlined above. If there is a mega amount of soil in the pool you can vacuum it to waste, if you have a multi-port valve. If you have a lot of sand or dirt and a D.E Filter, you should remove the grids or cartridges, put the lid back on, put the backwash valve to waste and vacuum the sand and small debris to waste. Inserting a leaf trap into your vacuum port or skimmer will allow you to vacuum up a lot more stuff without clogging your pump basket. In extreme situations, you sometimes just have to drain the pool and scoop it out by hand.
Problem #3 - green water due to metals.
Metals suspended in water can come from several sources, but usually green water is caused by copper. Copper can enter your water form a variety of sources - the addition of a copper algaecide, your water source, or by erosion of copper plumbing, especially your heater. Copper algaecides have been in use for decades, and it kills most types of algae. Though the product does have some negative properties that you need to be aware of. Because the copper is in a very fine particulate known as an ion, it readily becomes suspended in your water. In large dosages your water will turn green. Many copper algaecides will include a chelating agent which helps keep the ions in solution. Over a period of time, they lose their effectiveness and you must add a metal inhibitor. Orenda Scale & Metal Control is an industrial strength product that works great. It has no diphosphonic acid which can add to your phosphate levels. (Remember, phosphates are food for algae. Exactly what we're trying to avoid!) Sea Klears Metal Klear is another good product.
Problem #4 - green water caused by erosion / corrosion to plumbing and heaters.
This is a very serious problem that is caused mainly by water imbalance. Low calcium, alkalinity and pH are the most common scenario. When your water chemistry becomes too acidic it has the capabilities of dissolving metal and turning them into ions. This is the most common cause of heaters failing and leaking. If you have a very old pool you may have copper plumbing and an acidic condition that will corrode your plumbing which will result in leaks. You may also notice the handrails, light fixtures and stainless steel items turning black. When you get a high solution of copper, the prospects of it collecting and falling out of solution and onto the pools floor and walls is common, this is evident by green stains. You may notice the water turning green before it actually coagulates and drops to the floor.
Note: You should not be swimming in your pool with this chemical imbalance, as the pH and alkalinity are so low it will burn eyes and cause your skin to itch. Also, this is the cause of green hair (not too much chlorine as most people suspect).
If you suspect this is what is causing your green pool and you take your water sample in for testing you may find two different scenarios.
  1. The first is high copper and or iron levels in your test sample.
  2. The other is no metals present which means they either coagulated and are on the pool floor, in the filter, or were never present to begin with.
The most common cause of stains in pools are acidic conditions that have been shocked by a large addition of a high pH product like soda ash, liquid chlorine or chlorine shock treatments. You have two choices with a high metal content, treat it or drain it. To treat it add a chelant and bring your pH and alkalinity up slowly. You can drain a pool using a sump pump with a float switch. Do to liability issues it is recommended that you drain the water into a clean-out located on the side of the house which goes to a community sewer system. Do not pump water out onto your lawn, field or in a planter bed. The water will cause plants to die due to the salts and chemicals in the water. It will also leave all that stuff in the soil and it may make it difficult to grow anything. Do not drain the pool into a septic tank system as it will cause it to overflow.
Erosion due to velocity
This condition is caused by a pump that is not sized properly for your pool and plumbing. Many pumps over two horsepower (HP) will deliver flow rates over 100 gallons per minute (GPM), some 3 HP pumps will reach 160 GPM. Two inch schedule 40 PVC plumbing has a maximum flow rate at 80 GPM with a velocity of 8.2 feet per second (FPS). Most heaters with two inch manifolds can only accept a GPM of 120 GPM and this is with their internal flow bypass valves. The smaller versions with 1.5 inch manifolds can only accept 70 GPM. When your pump exceeds these maximum flow rates you increase the speed, FPS and flow GPM. This in turn forces water through the heater at such high velocity that the water will virtually erode the copper in the heater. If you have excessive flow rates, you can install a manual bypass valve before the heater to control the flow through the heater.
To learn how to determine your flow rate, read our article: How to Calculate Flow Rate. Using this information you can access one more chart called a friction and flow chart. This chart informs you of the velocity per second and the friction loss per 100 feet of pipe. Slowing down the speed of the water by increasing the pipe size is the desired outcome. Increasing the GPM in a smaller diameter pipe leads to erosion and pump damage. It is easy to state that the two most common causes of heater failure by leaking is caused by out of balanced water chemistry and erosion due to too much velocity, the combination of the two causes damage in a very short time.
We always advise that you do NOT to swim in a pool that is green or is going through the process of being cleaned up. No matter what is the cause of your green pool. Your water is not sanitized or balanced, and when you are cleaning it you will more than likely added chemicals in strong concentrations. You really need all the help you can get to clear up your water and getting in the pool will only hinder that process.
Are you having pool problems that you cannot resolve? Give us a call, we are here for you seven days a week.
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