You won't need a pump unless your tank is on the floor. Put a small section of 1/2 PVC pipe down to the bottom in one corner. Make sure to make a few small cuts in the bottom so water can flow into the inside of the PVC. Cover the top of this PVP above the level of your LECA and Soil mix with a cap and cleverly hide with leaf litter, wood whatever.
Now whenever your LECA layer starts to have water build up, use a length of flexible tubing, I use aquarium airline hose, to siphon water from the bottom by inserting the flexible tubing to the bottom of this PVC riser and starting a siphon to suck the water out.
This may be altogether unnessesary as you may not mist enough depending on your conditions for water to build up. Personally I have to mist 2-3x a day in the very arid environment of El Paso, Texas to maintain my humidity.
The other option, if you can, is to drill a hole in your tank, put in a bulkhead and a ball valve. Whenever you want to drain the water, just place a bucket under the valve, open, presto, drained. This is far more expensive but probably easier in the long run and you can do it without having to open the tank. Remember only certain tanks (non tempered glass) can be drilled. Bulkheads and fitting will set you back $10-$15 or so. Worth it IMO.