I will shortly post an overview on some of the currently available chemical testers, from the electronic ones to the colour matching ones and the merits and pricing. Buying off Ebay may seem cheap but they won't help with advice and if buying in from the USA by the time shipping and import duty have been paid the price is very similar but EU instruments have a warranty US units don''t. I don't propose an exhaustive list as I don't intend to get involved with units that do not meet basic requirements of cost and features. If you want a high end machine email me for more info.
With your fresh start up last year you are unlikely to have any CYA in your pool unless it was dosed in by Thermapool which I doubt. Each chlorine tablet (Multiaction Galet ) adds chlorine and some CYA, you need this to prevent the sun's UV from burning off the chlorine too quickly. It could take nearly a season to add the CYA this way and you will be losing chlorine all the time. The half life of chlorine in a pool without CYA is around 35mins so the chlorine is all gone after 2-3 hours that means algae can grow as you have found out now. To rid the pool of this algae you need more chlorine.
You must keep a residual level of chlorine in the pool at all times unless a winter (dark, light obscuring cover) is in place.
After a while using galets the CYA figure will reach 30-50 ppm (parts per million) at that point you have about the maximum protect CYA can give you but the residual level of chlorine need to also be increased slightly as most of the current chlorine will be chemically bound to the CYA so increasing the free available chlorine level to 1.5-2.5ppm is what is then needed to provide a residual to counter algae and CYA and keep the pool clear. Chlorine is still burnt up by the sun's UV and also after rain showers etc so must be replenished, this can be done in an erosion feeder (flying saucer shaped item in which you put chlorine tablets) this will allow the tablets to dissolve at a steady rate.
If you have a salt water chlorinated pool you still need CYA so can use the galets or a liquid or powder version of CYA dissolved separately into the pool but it will be needed or the cell plates producing the chlorine will be working harder to produce chlorine more frequently and they will wear out quicker and the cells are not cheap.
To shock a pool full of algae you don't need products called shock (Choc) you need to raise the chlorine level to a point where all the algae is killed off and that can take a fair bit of chlorine and some patience, as the chlorine will get used up quickly killing algae so it needs regular top ups to maintain the high level until the algae is dead and oxides/filtered out of the pool and the chlorine level remains in the pool steadily without dropping. We use the combined chlorine and over night chlorine tests to determine when shocking is complete.
It's not a spectator sport either, you need to run the pump and brush the side and bottom frequently and backwash the filters (rinse out the filters or bags if you have them).
The only plus point of no CYA is the chlorine is a bit more powerful without CYA so shocking a pool is easier but because your chlorine can get burned up quickly shocking in the evening when the sun isn't as strong is a good idea but if you have algae, do not wait get the chlorine in there asap but add more ad sun down.
A good cheap source of liquid chlorine is Eau de Javel in 20ltr biddons from a Brico shed, Brico Depot usually stock this for €12.90. Being a liquid it doesn't waste time dissolving and contains no CYA, this means it's great for dosing your pool all the time.