HP-a High Pressure tank, usually running off of 800 psi give or take
LP-a Low Pressure tank, usually running off of 400 psi give or take
Here is a list of HP guns-
LED
LCD
IR3
Here is a list of LP guns-
03/04 SPEED
A4
A4 FLY
05 SPEED
G7
G7 FLY
06 SPEED
ANGEL ONE
LP suggested, but may run fine on HP-
Angel One Fly
With that being said, HP guns take Hp tanks and LP guns take LP tanks. If you have a LP gun and run HP on it, you run the chance of it missing up orings, shims, the poppit, and many other things, so its best not to run HP on a LP gun. Also, you may not be able to get the velocity under 300 FPS. With that being said though, you may use a HP tank on these guns-G7,G7 fly, 06 speed, A1-If you do though, the consistancy will NOT be as great as with a LP tank.
The 05 SPEED and all the guns above the 05 SPEED(G7,G7 FLY,06 SPEED, ANGEL ONE) have a different reg and are supposed to let you use HP on them. Some of the 05 SPEED regs can not handle it and some can. I would go out and test your gun and see if you can get your velocity under 300 FPS so you do not have to do the following--
If you have a LP gun and have a HP tank you can do several things. One of which is get a CP reg or any reg that can accept a HP tank; you can also change the minireg to accept HP (I will give a detail example of how to at the bottom of this post); you can also ask the company who made your tank reg if they can make it LP.
If you have a HP tank you can modify the mini reg to lower the spring stack pressure so you may adjust the gun below 300 fps.
1) remove all pressure to the gun.
2) remove the mini reg from the gun by unscrewing it counter clockwise
3) look into the top of the mini reg and locate the circle 'c clip'
4) with a circlip pliers remove the circlip
5)carefuly turn the mini reg over and tap it on a padded surface
6) you will note that there are cuped washers stacked on top of each other they look like this: )()()(
7)remove 1 of the cuped washers (springs) from the mini reg stack

reassemble in reverse order and re chrony
Of course, it is always better to just go out and get a LP tank or see if you can change the tank reg to LP before you do any mods to your reg.
I hope that helps everyone asking questions on thier tank/angel. If anyone else has something to add, because Im sure I missed something, let me know and I will edit it.
-Josh
Edit - The Angel One Fly seems to be a mixed bag. Some people have loads of trouble trying to run HP tanks, and have to do the minireg mod. Some people run their HP tanks just fine with no modifications. Best I can say is if you've got one coming, or plan to buy a Fly, try your HP tank. If it doesn't work, either mod the reg, or swap your tank reg for an LP model. (PS)