Hi Nickdee,
Welcome to TFP!
I would like start by saying that its been proven 1000s of times, that pool store testing along with the advice they give varies wildly. I would highly recommend that you get your own reliable test kit, and test the water yourself. There are 2 recommended kits... the TF100 and the Taylor K2006. Both are awesome. The TF100 is be the best bang for the buck.
www.tftestkits.net is the only place to the TF100. THe K 2006 can be gotten there too, along with other places online.
Ok, so having said the above.
You might need to replace SOME of your pool water, but only IF the test results by the pool store are anywhere near accurate.
IF, the pool store test is anywhere near accurate... then...
Your CH of 600 is high, but manageable if you know how to manage it to prevent calcium scaling to your Pebble Tech plaster. (you need reliable tests to manage high CH) Replacing some water will lower the CH
Your CYA of 120 is not good. Definitely need to replace some water to get the CYA down. For a Chlorine pool, we recommend this level to be 40-50. With such CYA, its difficult to maintain enough Free Chlorine to fight off an algae bloom. The most common cause of high CYA is the use of Chlorine tablets. For every 10 ppm of FC a 3 inch tablet adds, it also adds 6 ppm of CYA. After a while, this climbing CYA become unmanageable because it doesnt go away. It climbs and climbs and climbs. Same goes for the powder shock products.
They either add lots of Calcium which drives up the CH or they add CYA.
The best source of problem free chlorine is actually bleach, or liquid shock.
Next, your PH at 8 is too high. You should get that down to the mid 7s by using Muriatic Acid.
Your TA is too high at 150, if you notice the Ph going up a lot and always having to add Murictic Acid to get it back down, then this is why.
You did not post what your
Free Chlorine value is ???
TDS is irrelevent if you keep your FC to CYA ratio in tact. (given that its at 120, its likely extremely low)
In order to help more, we really wold like to know more about your pool and pumping equipment. Whenever someone else comes along, they will know what you have. Knowing it does make a difference as to the suggestions offered.
If you will
fill out your signature, it will help us help you better.
here is a link to info and a link to edit your signature.
http://www.troublefreepool.com/content/115-read-before-you-post
Here are some useful references for your water parameters.
http://www.troublefreepool.com/content/134-recommended-levels
http://www.troublefreepool.com/content/128-chlorine-cya-chart-slam-shock
If you want to know how to efficiently and effectively maintain your pool, the clicking on the Pool School button at the top of the page will get you started. Lots of good info there, but at the very least, get your levels within the recommended ranges as quickly as you can.
TFP also provides a handy tool to know how much of what chemical to add so you can get to the recommendations. Pool Math Rocks!
http://www.troublefreepool.com/calc.html