The commercial products work very well and are worth the price. Both General Hydroponics and Advanced Nutrients make one. Clearex works ok for a flush but it's really meant to be used on a regular basis.

It's really up to you if you want to fulsh or not. If you use plain water it will take about 10-14 day's for a proper flush. The commercial products will flush in 1/2 that. Overwatering is not an issue at the end by the time the plant suffers you can chop it anyway. You flush to remove the the chemical residue giving a better taste to the over all finished product.