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There are a couple of issues that can cause this. One is the use of test strips. They can bleach out at high chlorine levels making you think you have no chlorine. You can check for that by diluting a pool water sample 3 to 1 with tap water and testing the mixture.
The more common cause is that something in the water is consuming your chlorine. It can be gone in just a few hours, so it's very common to put a bunch of chlorine into the pool at night and have nothing left by morning.
Pool stores try to get by with "one size fits all" advice that is seldom appropriate for a particular situation. We emphasize understanding so you can do exactly what YOUR pool needs. Fortunately that understanding not only lets you do a better job maintaining your water, it is cheaper and easier.
In order to really know what's going on, you need to do your own water testing, because the average pool store is not much better than rolling dice to get test results.
You want to know your actual CYA level, then add enough chlorine to the pool to reach an appropriate shock level. You can determine that from the
Chlorine CYA Chart Next comes the part that pool stores miss: Maintaining that shock level. We refer to the process for overcoming biological contamination of the pool as SLAM or Shock Level and Maintain to emphasize that ongoing nature of the process. You can read about it in the article
SLAMing Your Pool
Choosing your chlorine source depends on the rest of your water chemistry. All homeowner chlorine sources affect other areas of water chemistry. Powdered shock will either rapidly increase your CYA or your hardness. That's fine if you need to increase them, but not if they are already high. Bleach or liquid pool chlorine (the same thing in slightly different concentrations) add salt to the pool, but in most cases that is a desirable side-effect, so we tend to favor the liquid.
To really have knowledge of your pool, you need your own test kit, and the cheap ones handicap you in ways you probably don't understand yet. We recommend either the Taylor K2006 (not the 2005) or the TF-100. You can buy either from TFTestkits.net. It will be the best investment you can make to make your pool care better and easier. It will pay for itself before this season is over.
Skip the phosphate remover for now. 99.99% chance it was a complete waste of money. Pool stores like to sell magic elixirs to fix problems that don't exist.