I have 2 water tanks installed in my 3 story building. 1st one is on top of the 3rd floor, the other one is on top of 2nd. The pump is installed at Ground Floor. Now I made one outlet at 800 lts of 3rd Floor tank of 1000 lts capacity as inlet for 2nd floor tank. However the pressure was very low and before 2nd floor tank filling, 3rd floor tank was getting overflowed. How to address this problem
The solution is to mount the tanks so the filled surface of each tank is or will be at the same level.
That means some tanks will be mounted on supports at different heights to others.
if These tanks are to stay on separate floors, then the second floor tank will need a one way connection to the outlet to stop the top tank filling it.
Both tanks need a fill control valve.
Can you make the lower tank pressure-tight? Make two connections, one connecting the bottom of each tank, and this should be large enough for water to move - it is the water pipe. The other connects the top of each tank, and is to prevent vapor-lock of the lower tank, this line can be quite small.
The lower tank will now be 100% full and under pressure, which may help your pressure problem. Its pressure will benefit from the "head" up to the upper tank. "head" is the weight of the water above the tap.
Read this answer for a description of a similar installation.
The best mitigation for this problem is to replace the line between the tanks with a much larger pipe. Since the drain should begin to operate as soon as the water level reaches it, there is almost no pressure available to drive the flow. The drain pipe should look like a drain pipe, not like a fill pipe.
Another mitigation is to restrict the pump flow to something the drain can handle. Either use a smaller pump, or install a gate valve or globe valve in the fill line. (Do not use a ball valve, these are hard to adjust.)
In short, to avoid flooding, you can make the drain larger or make the fill smaller.
If you don't mind making your project more complicated, you can install a safety cutoff float valve in the upper tank. This switch must trip at a level above the drain but below the top of the tank. The pump must stop when the safety switch is raised. It will start again when the drain has caught up.