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Question:
We live in Orlando Florida, and are relatively new to owning a pool.  Ours is an
in-ground somewhat kidney shaped pool, approximately 17x32.  In the last 2
days, we have been fighting a problem with Algae.  The one we have was
"diagnosed" as mustard algae by our local pool supply store.  It is yellowish,
but looks green when brushed away, and tends to form into "piles" on the
bottom of the pool in yellow clumps.  We have a tablet chlorinator on the filter
system, the filter is new, and we use liquid chlorine.  We recently installed a
pool heater as well, and are wondering if the heat is contributing to the algae
problem, as we have never had it this bad before.  The pool supply store
recommended using Muriatic Acid, the liquid chlorine, and the chlorine tablets,
and said if we did the treatments today, we could swim in 24 hours.  My husband
actually had put the liquid chlorine in last night, and we used the muriatic acid
today.  The store said you could use the pool after 2 hours as far as the muriatic
acid was concerned.  This evening, I got into the pool to see if the chemicals
would burn or cause me any problems before I let my grandchildren in.  I did
have some burning sensations, not bad, but enough to keep me from allowing
the children to get into the pool.  I am wondering if we are getting correct
information from our pool supply store, and are hoping you will give us additional
(or correct) information, as to what we need to be doing.

Response:
What you describe does indeed sound like mustard algae.  Now..., I hope that
you've had a proper water analysis done.  I don't like to hear that pool stores
have you adding muriatic acid on a "willy-nilly" basis.  That's only going to
treat the symptoms & not the actual cause. Mustard algae is difficult to deal
with but is treatable.

If you're feeling that the water is uncomfortable, the water may be severely
out of balance.  If you can't find a good pool store to do a proper analysis,
at least test it at home regularly & maintain a pH of 7.4 - 7.6, total alkalinity
of 125 ppm & calcium Hardness of 200 ppm.  Algae is much easier to treat &
cure when the water is in good balance.  Furthermore, using a product such
as BioGuard Optimizer Plus at the correct levels will virtually eliminate any
algae problems. Period.
 

Question:
You mention small cell green algae (SCGA), which it is resistant to chlorine and
seems to make chlorine "disappear" and that sounds like what is happening in
my pool this season.  I've used many bags of powdered shock, gallons of liquid
chlorinator and gallons of an algaecide/clarifier.  In the past few weeks, I've
spent big bucks on the above; have vacuumed daily both through the filter
and to waste.  The water is still not very clear, and it eats any chlorine I add
in a matter of hours.

Response:
Constant algae blooms are a sign that you have a chlorine demand problem; in
other words, you're just not getting enough chlorine in the water at one time to
kill off all of the "stuff", no matter what it is. Keep in mind that when you use
liquid chlorine & gallon algaecide, you're using products that are pretty well diluted. 
When treating situations such as yours, always use the most concentrated products
you can.  They may cost more initially, but they'll do a better, more thorough job &
save you money in the long run.

Back to your chlorine demand problem.  Depending on your pool size, you'll
probably START with using 8 to 10 lbs. of shock (BioGuard Burn Out Extreme)
per 10,000 gallons PLUS a good quality algaecide (BioGuard Back Up or Algae
All 60).  That may sound like a lot, but trust me, it's not.  Using Optimizer Plus
will also further help because of the chemical reaction; the algae will not be able
to photosynthesize, when used in the proper dosages. SCGA needs to be treated
AGGRESSIVELY because it's very tough to kill.  My advice to you would be to find
a local BioGuard dealer who has the AccuDemand 30 chlorine demand test station. 
They'll be able to properly guide you.  If you can't find a dealer, you can overnight
a 1 qt. water sample & we can perform that test for you & make the necessary
recommendations.  Make sure that the water is properly balanced first.  That
allows the chlorine to work properly & effectively.

More Algae Q & A here (page 3)