Yes, use bleach. If your CYA level is correctly measured you must not use either "stabilized Tabs" (Tri-chlor) or "Di-chlor" powder. Bleach or Liquid Chlorine is your best bet.
If you Calcium Hardness or Total Hardness level is low (Below 200 - 300) you can also use Cal-Hypo powder, sometimes sold as packets of "Shock".
But do not use Tri-Chlor or Di-Chlor as they will make your CYA levels worse--anything over 100ppm means a different tactic for maintaining a clean pool. (or you drain off half your water and refill).
But bleach or LC is best.

Now: How much to use? There's two parts to this question:
1) How much chlorine will a gallon of bleach/LC add to my pool?
2) How much chlorine do I need?

1) There are two ways to figure the level of chlorine added.
You can either find, download and use the "Bleach Calculator", which works very well, but someone else will have to give you the link. or..
You can use the Rule of Thumb (which is based on the same formula). I like the Rule of Thumb because I can approximate easily how much in my head.
It goes like this:
One gallon of regular bleach (5.25%) will add exactly 5.25ppm of free chlorine to 10,000 gallons of water.
One gallon of 6% will add 6ppm to the same 10,000 gallons, and so on (12.5% LC will add 12.5ppm)
But if you have a 20,000 gallons, it will only add half that much--so 5.25% will only add 2.625ppm, so you need TWO gallons to get the same effect.

You simply adjust that way.

Now with 18,000 gallons it seems tough, but it's not. You can approximate. You can either figure your pool at 17,500 gallons which 1 3/4 times our 10,000 model, or go all the way and round up to 20,000 gallons. Since you have a plaster pool there's no danger of damaging the pool if your chlorine levels are a bit higher than the Rule says. I have a 19,000 vinyl and I just use 20,000 with no problem.

So...to raise your free chlorine by 6ppm, figure 2 gallons of Ultra-Bleach (6% concentration).

2) How much do you need?

With a CYA of 170, the recommended level for Free Chlorine is between 8 and 15ppm (I'd stay to the higher side of that, but you are just at the low end of the target) and if you have algae you shock the pool by raising it to 25ppm. Since you have a Free Chlorine level of 8, you'd need to raise it by 17ppm. Since every gallon of 6% ultra adds 3ppm, you need 5 or 6 gallons. Since you REALLY have 18,000 gallons, the 5 gallons should be enough to reach 25ppm.

If your pool is clear at 8ppm, I'd still raise it to, say 11 by adding a gallon of 6%

While none of this is exact, it's actually good enough to clear your pool and keep it clear. I rarely sit down and work the formula, because I worked it out for the 10,000 gallons and can adjust as needed.
If you want to get crazy there are ways to measure exactly the strength of the bleach or LC (I get the LC from a place that nominally sells it as 12.5% but when I test it I get 14% so I need a little less).

I hope this helps.

Carl