Spot etching treatment and prevention

[jamieSATX]

We have a 13,000 gallon plaster in-ground pool/spa combo that was built in 2010. We've been in the house for a year and - thanks to everything I've learned from the forum - I've been meticulously maintaining the pool and water chemistry.

The pH tends to run high because of the spa spillway and also a large waterfall. To balance the pH and keep it below 7.8, I need to add muriatic acid quite frequently - almost every other day in summer when the waterfall is on. In the summer, I also need to add about 20 oz of liquid chlorine per day to maintain the chlorine level close to 4-5 ppm. If the alkalinity runs below 85 ppm, I add baking soda. CYA is 40 ppm and CH is around 320 ppm.

Despite meticulously maintaining the water chemistry, I just noticed what appears to be some new spot etching on the plaster in the deeper end of the pool where I add the chemicals. I'm trying to figure out what could have caused that and how to prevent it. When adding muriatic acid, I fill up a 5 gallon bucket about half full and then dilute the acid in the bucket before adding it to the pool (never more than 25 oz at a time). I don't dilute the liquid chlorine or baking soda before adding it. Pump is always on high when I add chemicals.

Do you think the spot etching could be from when I pour the muriatic acid into the pool? Not sure what I could be doing wrong. The overall pH never tests below 7.5.

The pool has never been replastered and we're trying to put that off as long as we can due to costs. Not sure if there's anything that could help prevent the etching and also remove the stains? I've seen the Jack's Magic Stain Solution #1 but am not sure if TFP recommends that product. Would a chlorine wash or acid wash help?

Picture of some of the etching is attached. There are some additional green stains, which are probably copper and have been there for a while. Not as concerned about those for now.

Thanks for any advice you have!

Plaster
13,000 gallons
Pool/spa combo
FC and TC 3.5
pH 7.6
TA 100
CH 320
CYA 40
Pentair quad cartridge filter
Pentair Intelliflo Variable Speed Pump installed last year
Pentair MasterTemp 400 BTU heater

 

[YippeeSkippy]

Your first mention of your TA being so high is the reason you're playing "chase the numbers" with your pH. The higher your TA is the faster your pH rises. You can use Muriatic Acid to bring your pH down, with the benefit of it *also* taking your TA down too. Then you let that TA stay between 60-80 and your pH will slow down. No need for baking soda!!

Lower Total Alkalinity

A lower Total Alkalinity level can help a pool owner stabalize their pH increase. We'll explain how to lower TA with a little help from muriatic acid.

www.troublefreepool.com

Next, please don't dilute the muriatic acid. That raises chance of accident. The best way to add acid is to lay the open jug of acid into the pool water's return and pour a thin stream into the moving water. You can guestimate how much you need, you usually don't mess up. I always brush well all around the wall and floor afterwards just to know nothing settled on that area.

Maddie

 

[cdadams1223]

Based on your testing, your FC is borderline low considering your CYA of 40. You're FC level should be 5-7 with an absolute minimum of 3. You can find this information on FC/CYA Levels by plugging in your CYA value. If it were me, I'd keep my FC at the high end of the range considering you're in Texas (as am I) with the sun beating down daily. As @YippeeSkippy mentioned above, don't dilute your MA. Just do as she says and gently pour it in with a pencil sized stream next to one of your returns and then brush the bottom of your pool, at least in that general area. You'll rarely need to add baking soda unless your TA falls below 50.

How are you testing your water? Please fill out your signature so everyone knows what your working with. See the following article: Create Your Signature - Further Reading

 

[jamieSATX]

Thanks to both of you! I'll change the way that I add muriatic acid since that seems to have been causing the spot etching issues. Moving forward, I'll also be sure to brush the bottom and wall of the pool after adding the muriatic acid, as I wasn't doing that before. I'll also aim to keep the alkalnity slightly lower (and chlorine higher).

I use a digital LaMotte kit to test the water almost daily. I also occasionally take it to a pool store to get it tested once or twice a month.

13K gal, plaster, Pentair Intelliflo Variable Speed Pump, Pentair Quad Cartridge filter, Pentair MasterTemp 400K BTU, Liquid chlorine, Polaris 9550 robot, LaMotte ColorQ 2X Home Pool 7 Test Kit

 

[mknauss]

I use a digital LaMotte kit to test the water almost daily.

Pool Test Kits - Further Reading

www.troublefreepool.com