Let me save you some money here. Go to a hardware store, and buy a waterbed filling adapter (which is the piece that connects to your sink). Buy as much clear vinyl hose as you need, and then get a few garden hose type fittings and put them on the hose. For $10 (depending on how much hose you need) you can make your own python, minus the gravel vac (which is junk...too little suction, and wastes a TON of water). Since a real python system will set you back $40, this is a great money saver.
I suggest then, also, to make an upside down U-shaped filling adapter out of
PVC. Get 1"
PVC, and 2 90degree elbows, a threaded adapter, and an end cap. Make a U shaped piece that fits over the edge
fo the tank. Put an end cap on the 'inside' and the threaded end on the outside (which you can attach to the end of your
DIY python). Glue the pieces together.
Then drill a bunch of small (3/16 drill bit) holes in the 'inside' end of the U-shaped filler.
I'll post a photo...it'll make more sense then. But basically doing this you can fill a tank all by yourself and won't have to worry about the hose coming out of the tank.
When I fill my tank, when I turn on the tap, I add my Prime to the tank, near the filling point. I have yet to have any fish die, or notice any plant problems. As AtodaJ points out...its prolonged exposure that causes problems.