How do I lower the water level in my pool? Show a diagram, so I can see what values I need to turn and adjust. My pool has a float mechanism and a drain fuction at the main control panel but I am not sure if it is used to drain the spa only (or spa into the pool).

It is the mechanism in the tank. Take off lid of tank flush toilet and watch what happens.
Does the tank have what looks like a float ball or float cup?
If you have the cup does the little water-hose align with the pipe it runs water into?
Does the cup move down and up easily?
Do you have a chain on the handle that needs adjusting?
There are lots of reasons water lowers in toilet. Need more info you can put in comment tab instead of posting another start post.
Here is a link just as a start once you have an idea there are many help videos to show you what to do. Why Is There Low Water Level In The Toilet Bowl And How to Repair

1. Check the wheel RPM. If not in the 30-36 RPM range, refer to the manual for adjusting the wheel RPM.

2. Adjust the thrust jet downward.

3. Verify the backup valve is cycling. Lift the valve out of the water and watch the jet. It should come on and go off.

4. Check for water inside the head float.

5. Empty the filter bag.

You can download the 3900 Manual from the Polaris site
http://www.polarispool.com/Pressure/Polaris-3900.aspx

It will show you how to check the wheel rpm and how to adjust the thrust jet and check and adjust the float.

Inside the tank, on a lever arm there should be a cylindrical float that rises and lowers with the water level. Attached to the float is a float rod with a clip that can slide up and down the rod. You can adjust the water level by sliding the clip down to reduce the amount of water entering the tank. As the water fills the tank, the float rises, pushing up the float rod, which in turn pushes up the level that controls the valve allowing water to enter the toilet.

Go to mftr web site to get a break down of fill valve it should show how to adjust water level,or contact co.and ask for tech support they should be able to tell you how to adjust water level or e-mail you a instructional diagram of how to adjust.

Hi there terminator,

Inside the tank, directly above where the water hose is attached, is the valve mechanism. Attached to this valve there is a float.

As the water level rises so does the float. When it get high enough, the float valve shuts off the water.

The idea is to lower the float so the water is turned off sooner.

If your model has a ball on a metal arm, it is as simple as bending the arm down a little bit.

Vertical cylinder types have adjustments on them.

Then there is the simplest solution which is to put a couple of bottles full of water in the tank to take up space. Therefore less water is needed to fill it...

Best regards

Mike

Hello...

To adjust the water level, you need to set the height at which the float shuts off the water valve. There are several types of floats. In older toilets, the float hangs at the end of an armature, along a metal or plastic rod. Where that arm connects to the water valve there is usually a screw. Adjusting that screw will change the level at which the water valve shuts off. If there is no screw or it is already at its maximum, you may be able to bend the metal arm, but you should remove it from the valve or risk damaging the fill valve.
In newer toilets the float is often a collar around the fill valve and rides up and down vertically. The float typically has a spring clip connecting it to a metal rod which connects to the valve shut off control. Squeeze the spring clip to slide the float up or down the metal rod.
Manufacturers have several variations on the fill valve mechanism, but they all include a method for adjusting the water level. Close inspection should make it clear how to adjust your particular fill valve.

I hope this helps!

There's no certain way for me to know which fill valve you have in there, but locate the float. This floats on the water level and turns off the water when the tank is full. There's probably a screw on the float arm that lets you adjust the float up and down, adjusting the full water level. You may be able to loosen a collar on the bottom of the fill assembly and slide the whole thing up and down (make sure you turn off the water valve when you do this because the whole thing can pull apart), or you may be able to simply turn the entire fill assembly which will raise and lower it if it's built that way. Make sure the new water level doesn't go above the level of the refill tube or the water will go down the tube and the valve will never turn off.

The flapper is not sealing. The water level could be set to high causing the water to overflow into the fill tube and you will need to lower the float. you will not need any parts to adjust the water level and the flapper is under $10.00. Click on the below link to see the parts diagram. http://www.hometips.com/how-it-works/toilet-plumbing-diagram.html

Hello, W/D, toilet expert, here.
The tank has a water level line cast into the inside back of the tank. That's where your water level should ideally be. Now, there are a variety of water level controllers, so we'll hit on the major ones. But first, a little nomenclature, so you'll know what it isn't. If you remove the tank lid, you'll notice a tube in the middle that's open, and usually has a smaller tube inserted or attached to it. This is the overflow tube, and feeds water to the rim of the bowl. At the base of the overflow tube is the flapper, and it controls the majority of the flushing power. The flapper is operated by a chain connected to the flush lever. When the flapper is raised (flushed), the water level drops. On the left side of the tank is the water fill valve, The water coming from the house via the tubing under the tank goes into this valve assembly. Part of this assembly is the all important float.
Older ones have the float at the end of an arm, As the water level falls, the float falls and operates the water inlet valve at the other end of the float, on the water fill valve. As the level rises in the tank, the float rises with the level and shuts off the water when it gets to it's pre-set level. This type is adjusted by positioning the float either by the screw adjustments on each end of the float arm, or sometimes by bending the float arm.
The newer types have the float either built in, or the float surrounds the fill valve. The one that surrounds the fill valve looks like a very fat donut and is very similar to the older float arm type, just more compact. The water level is set by adjusting the linkage between the float and the water valve at the top of the water control valve.
The newer newer type has all of the floating mechanism built in. But cleverly, there is an external adjustment for raising and lowering the water level. On the side of the tube, there are slots and raised dots on the lower tube. The upper tube has slots on the inside of it. There is a white locking tab between the two tubes. Here's how this one works: (Turn your water off first for adjusting this type). The tube is in two parts, an upper and lower tube. They are held in place by the locking tab, and by the slots cast into the two tubes (they intermesh) To adjust this type, you have to raise the lower portion of the locking tab to raise it off of the locking dots, and than revove it. Then, you push down slightly and twist the upper section to unlock it. Set the level higher or lower, twist the two parts back together, and reinsert the locking tab. The aim is to set the water level at the level mark on the tank. This type can be tricky; you'd do best to turn the water off to adjust this type, and turn it back on once you have locked it back in place.
Best regards, W/D