whoa, whoa, whoa....put down the electrical tape. Step away from the electical tape there sparky....
Sit down class, the frogmans gonna tell you a story 'bout a cool cat named Mr. Ohm. Mr. Ohm was a bad dude, that was so good with da electricals that the man made his own law. Da brotha's law is called Ohm's Law. Mr. Ohm's law had stated that when voltage is decreased (via resistance) amperage increases. He goes on to say, lower the voltage, higher the increase in amperage draw in the presence of the increasing resistance. Pickin' up what I'm puttin down? Lemme break it down for ya's jive turkeys... 8)
Well say as given, your battery is 12volts, the pump draws 10amps continually.
*You want to slow the pump down because it's pushing too much water
*The only way to do this is lower the voltage to the pump
* In order to lower the voltage, you must add resistance to the circut (either a resistor or a rehostat[dimmer kinda knob])
* When you add resistance to the circut, you will lower the voltage, the pump turns slower, less water is pushed, HOWEVER now the pump is drawing more amperage (juice) because the voltage has been lowered.
So class, what have we learned from the law. Oh, Oh, Oh from the back... "two men enter, one man leaves. two men enter, one man leaves." #-o Nope, wrong law. We have learned (all things equal, in a perfect world) that the motor that was once too strong, that used to require 10 amps of juice to run wide open, now requres considerably more juice to run at half speed ...and you can now kiss yer battery life good bye. Tinboats here!...we need as much life outta a battery as we can get.
Back to Ohms Law again....low voltage resistors are astronomically more robust and expensive over 120v AC resistors...lower the voltage, higher the aperage...more amps=a ton of heat....Like, poof, small fire, let out the majic smoke heat. And, as soon as you let out the magic smoke, you can't put it back in. A ceiling fan rehostat is made to operate at 120v (10x) the voltage of the 12v pump. Adding resistance to a 120v circut [like a dimmer on a fan] isn't going to even tickle the amperage or juice drawn by that fan. Drop it down to 12v [like the pump] the amperage is going to multiply expodentially. If you put ceiling fan dimmer on a 12v circut (granted if you can actually close the circut at 12v), you're going to blow out that switch so fast, the human body will be incapable of seeing the flash of the switch internals frying.
Now, if you wish to lower the voltage on that pump, yer gonna have to build some circut stuff. Two ways of doing this...rheostat (potentometer) {jeez do I wish the spell check was working} or a plain old resistor. A rheostat will alow you to adjust the voltage up/down variable...kinda like a gas pedal in a car. A stand alone resistor will cut the voltage to one level, slow down the pump, and leave it at one speed. Sounds easy right?...nope. Anything from a light bulb to a motor on a 12v system is pulling OMG levels of amperage electrical world speeking. OMG levels of amps requires really, really big resistors and rehostats. The standard Radio Shack/ Jameco, "grain of rice" resistors and bread board rehostats are just gonna puff smoke like a Lionel Train locomotive. If your dead set on a rehostat, start looking at supply warehouses for something off of industrial lift trucks, or real locomotives...otherwise it's gonna go "poof" in a couple of uses. The "stand alone resistor" way, off the top of my head, without doing the math, you'll need 500Kohms to 1Mohms to cut the speed in half, and you're going to need a big "ballast resistor". Automotive use ones aren't good enough, they still need massive airflow to keep them from buring up....back to the supply shop, where you'll need to find another lift truck/locomotive application, open air cooled, size of a beer can, expensive resistor to run that circut reliably.
In conclusion, buy a smaller dispalcement pump. Don't mess with the LAW! 8)
