What are the possible causes of green algae entering an indoor pool when all chemistry is balanced, disinfectant levels adequate and UV system and filtration functioning?
Is algae always introduced from outside sources, such as equipment that has been in open water environments?
We have shocked the pool, it went away. It is back again.
Algae is actually floating in the air (Even inside). It can also be brought in on equipment and swim suits. It is basically ubiquitous. Algae in an indoor pool is very possible, especially in an area with a lot of light and low sanitizer levels.
Chlorine, at pool levels, doesn't really kill all algae, but inhibits its growth. Most types of algae grow in biofilms on the surface of your pool, so UV light can be of limited usefulness. Brushing the pool surfaces, to break up the biofilm, can help during the treatment process. Treat your pool equipment when you treat the pool. If appropriate, just throw the brushes, vacuum hoses/heads/poles, lane lines, maybe even rescue tubes, etc. into the pool to soak during the pool treatment.
I would superchlorinate the pool, and give it an algaecide treatment, depending on the type of algae (consult with a reputable pool dealer for the best type of algaecide for your type of algae). Black algae will need spot treatments and lots of brushing, and may never completely go away (it grows right into the pool surface). Cleaning your filter with a filter cleaner (not acid) and superchlorinating the filter can help avoid reintroducing the algae back into the pool (it certainly won't hurt to clean your sand, is appropriate for your DE socks, and does wonders for your cartridge filters.). After cleaning the filter(s), it can be treated with acid, if necessary, to remove any mineral buildup.
Once it is you are back to normal operation, start testing the chlorine levels at various points around the pool. You may find that the pool has dead spots where there is little sanitizer. Adjusting the inlet fittings can help get rid of these areas.
A dye test can help to identify these dead areas. The dyes used for pool testing only last a few hours before they disappear. Dissolve a dye tab or two (follow the directions) in a bucket of water and slowly pour into the surge tank or a drop a tab into a skimmer. You want the dye to slowly feed into the pool through the inlet fittings. You can fairly easily see where you have dead spots and short circuits in the flow pattern. You will also often see significant differences in the flow through different inlets (some variation is normal, it is very difficult to get all working exactly the same. Look for large differences.)
Periodic additions of a good algaecide is often beneficial and certainly won't cause problems. Another option can be to eliminate the phosphates to remove a food source and stop the algae from growing. The best solution is finding out why your chlorine isn't enough to stop the algae growth. Maintaining adequate levels is your best insurance.
Thank you so much for your reply.