Why is the water in the freshly filled above ground pool green

2000 gals is a lot for a couple of 1" tabs to chlorinate. I put one to two tablets into a 350-gallon hot tub and sometimes even that isn't enough to maintain proper chlorination. I'd be looking at one to two 3" tablets for pretty much anything you'd call a "pool".

The short answer to your question is, yes, you want to put a "start-up" dose of chlorinating agent into your pool to get it to the proper chlorine level (or slightly higher); from there, a "maintenance" dose of chlorine such as from the tablets will keep the proper level.

The longer answer is that there is more to conditioning a pool than chlorinating it. The chemistry beneath the surface has been the subject of many a doctoral dissertation, but the products available commercially, while not idiot-proof, are hard to get wrong if you follow the instructions. The steps you follow, in order, are:

1. Balance pH - The water should generally be slightly higher pH than neutral, generally between 7.2 and 7.6. Too low and you risk an acidic environment which can damage metal parts like your pump. Too high and the water will cause ichy, dry skin (or in the extreme, chemical burns) because the high pH breaks down natural body oils (a process called saponification, which is the same as is used to make soap).

   The typical "pH Down" (acidifying) product for home use is sodium bisulfate, NaHSO₄, which is an "acid salt" based on a partially-neutralized strong acid (sulfuric), which works as a weak acid in water. This product, in powder form, is safer to use and store than liquid muriatic (hydrochloric) acid, HCl, which is commonly used in large public pools where you don't have time to mess around with dissolving several pounds of a powdered product. Muriatic acid also reduces total alkalinity (useful if your water comes from an aquifer, less so if you're already fighting soft water from a marsh or river). "pH Up" is typically a weak base, such as sodium carbonate or sodium bicarbonate (baking soda), while lye (sodium hydroxide) is used commercially; sodium bicarbonate also increases total alkalinity.

   Work in small amounts and let the chemical dissolve and mix thoroughly in the water as you go. Also, never pour acidifying products directly into your skimmer; the concentration needed to balance a full pool will make the water in the circulation loop highly acidic for long enough to cause serious damage to your pump's impeller and other circulation parts. Alkaline products, not so much; the alkali metal cation already attached to the hydroxide or carbonate anion is more attractive than the transition metals of the impeller, heater etc, so these are typically skimmer-safe. Just don't ever put any balancing product (any maintenance product, really) in the water while anyone's in it.

2. Buffer - You need the right amount of alkali metal in the water, and of a "chlorine buffer". Basically, you want the water "hard", but not so hard that you have to deal with scaling on a regular basis. Chloride salts, like calcium and magnesium chloride, both add some chlorine to the water, and more importantly provide a pH buffer with the alkali metal, giving acidic compounds (like many chlorinating compounds) something to dissolve besides the metal parts in your pump. Sodium bicarbonate (baking soda), in addition to increasing pH, also increases total alkalinity. Too little alkalinity can cause large swings in pH as chlorinating agent is added and then burns off. Too much alkalinity raises pH, reduces the amount of "free" chlorine that can actually do the disinfecting work, and causes cloudy water, scaling, and itchy dry skin.

   As for the chlorine buffer (often called a "stabilizer"), cyanuric acid is generally used. Cyanuric acid reacts with chlorine to form stable "chlorinated cyanurates", which are good disinfecting agents and also prevent the chlorine being lost from the water as gaseous chloramines. These products are generally skimmer-safe; you may avoid putting straight cyanuric acid in the skimmer, but cyanuric acid is a weaker acid than hydrochloric (enough that it has a negligible net effect on pH when added to properly-balanced water). Watch the amount of cyanuric acid you put in; unlike the chlorine compounds, cyanuric acid is ridiculously stable and will not "boil off" (which is why it's used), and won't readily react with the alkalinity in the water, so too much of it cannot be negated by adding something else; the only way to get rid of it is a partial drain and refill to dilute it.

3. Shock - The general goal of "shocking" the pool is to temporarily superchlorinate it, which kills bacteria and algae; the normal level of chlorination is a deterrent to bacterial growth, but certain nasties are chlorine-resistant, and the amount needed to guarantee the water is sanitary is a bit much for humans (and their swimsuits). Shock is also designed to "burn off" relatively quickly, carrying the organic byproducts of bacterial destruction with them (that "chlorine smell" of a pool is the smell of organic chloramines evaporating, the result of the shock doing its job to clean the water). The end result should be clear, blue water, free of chloramines which turn the water green, and with a free chlorine level (unattached Cl^- ions) of between 3-5ppm.

   A variety of actual chemicals are used to shock water; sodium hypochlorite (chlorine bleach) and the related calcium hypochlorite are definitely effective, but tend to have detrimental effects on swimmers' skin, hair and swimsuits, and the distinctive "bleach smell" which is good for laundry, bad for swimming. Many powdered shocks have more complex, tailored chlorinating chemicals that act as disinfectants but not bleaches. Many powdered shock compounds also include a "clarifier", which is a substance that causes small particles (which normally have a small negative charge and repel each other) to group together into bigger particles, so they won't remain suspended in the water and can get caught in the filter. Most shocks also include a dedicated algaecide (see below). Shock is almost always skimmer-safe, but chlorinating agents can lower pH if there is an insufficient amount of an alkali buffer.

   Once these three steps are performed, dip a test strip and verify that the water has the proper pH, hardness/alkalinity, total chlorine, and free chlorine. If there is too much chlorine and slightly low pH immediately after adding the shock, that's normal; within a couple hours of sunlight the chlorine level should go down to swimmable levels (and the pH should go up as the chlorine ions are removed). If there's WAY too much you added too much shock; make sure the pH isn't too acidic (still above 7; add some baking soda to counteract the acidity if it's too low, and you'll need to keep an eye on the pH balance) and keep people out of the water until free chlorine comes back down to 3-5ppm. There are "shock and swim" products designed to be swim-safe very soon after adding them; these usually use a "self-buffering" chlorinating chemical that reduces pH swings and the severity of the free chlorine spike, to prevent harmful levels of free chlorine while still being enough to "shock" the water free of contaminants.

4. Chlorinate - Most of the chlorine in your water at any given time should be added by shocking it. However, shock is generally short-lived and the amount of chlorine in the water will rapidly decrease after shock is added. To combat the loss of chlorine and stabilize the total/free chlorine levels, you use chlorine tablets, which dissolve in a time-release manner to add chlorine back into the system at a relatively constant rate. The number/size of chlorine tablets you need depends on the size of your pool, whether it normally stays covered or open, how much cyanuric acid you have, whether you have a dispenser (which further controls the amount of chlorine released into the pool by controlling the flow of water into the dispenser chamber with the dissolving tablets) etc. Like I said, chlorine tablets are not a replacement for shock and vice-versa. A chlorine tablet in the skimmer won't harm anything, but it may cause the tablet to dissolve too quickly, over-chlorinating the pool.

5. Algaecide - Included in many shock products and also available separately, the typical algaecide is a quaternary sanitizer that is more effective against multi-cellular algaes than chlorine alone, and is generally less harmful to humans (quat sanitizers are food-safe where bleach is not). If the test strip says the water's good, but the pool walls feel slimy, a dose of algaecide is usually the answer. Too much algaecide can cause a loss of chlorine (quaternary sanitizers are based on ammonia, and those chloramine gases I mentioned earlier that cause "burn-off" are chlorinated ammonium compounds) and can also mess with pH (remove chlorine from a balanced mixture, and the alkali metals left behind will react with the water itself to form hydroxides, raising pH). Despite this, algaecide is generally skimmer-safe.

Green is most likely algae, but it could be metals. You have to get your water tested for metals and phosphates (most pool stores will do this for free if you bring in a water sample), and then get rid of any measurable amounts using products designed for the purpose.

Cloudiness is also consistent with both of those problems, but could be independent: If after you resolve any algae and/or metal problem it's still cloudy then add a clarifier. If that doesn't solve cloudiness then you would most likely have a filter problem – e.g., tear or leak in the filter.