I've been having a PM conversation with NEMES1S about pony bottles, and thought it might be useful to upload a few pics of how I rig my pony. I prefer a slung pony rather than tank mounted for a whole raft of reasons, but mainly because it's easier to see the whole rig and know what is going.
Firstly, here's a photo of my normal pony bottle fully rigged:
The clips are used to attach the pony to the shoulder and hip D rings on my harness. The rig is oriented so that the valve points down, and is easily accessible to turn on and off. The reg is stowed by using a piece of bungy, and then trapping the reg mouthpiece pointing downwards to reduce the chance of a free flow, like this:
This particular rig has a small button gauge - not ideal, but I recently flooded the reg and trashed the SPG and haven't replaced it yet. Some means of monitoring tank pressure is essential, though. My preferred option is an SPG on a 6" hose. Here's a picture of one of my deco tank rigs that shows the SPG:
The SPG is orientated so it's easy to see when the tank is clipped off. It's held in place by, in this case, a cable tie that holds the short hose in a loop. I'm probably going to replace the cable tie with some nylon line once I've finished writing this!
In both of these photos you can see how the stage/rigging kit is attached to the top of the tank. The kit is essentially a loop of 5.5mm nylon line from a climbing shop. In this case, I've used a "larks foot" to loop around the tank valve. This isn't really necessary, and many people will just put a plain loop around the valve. The first clip is then tied into the kit using a simple overhand not. The position of the top clip is quite important - it should be around about level with the shoulder of the tank, with no excess cord so that it's nice and snug. This brings the tank up close when worn.
To secure the stage/rigging kit to the lower end of the tank, I use a small tank band as shown in this picture:
You can use a jubliee clip if you prefer. I use a tank band because it makes it easy to swap stage kits around the various pony and deco tanks that I use. They also don't rust - important if you have a bare aluminium tank like mine, as stainless steel in contact with aluminimum in salt water is a recipie for all sorts of corrosion problems!
This is what the tail end of the stage kit looks like:
The knot in the cord is a double fishermans, which just produces a loop. The second clip is again held in that loop with a larks foot.
A couple of factors will influence how the tank sits when you are in correct trim, namely where you put the tank band and the length of the tail of the stage kit. It's easy to control the length of the tail by wrapping it around itself like this:
The tank band (or jubliee clip) should be about 2/3 to 3/4 down the length of the tank, depending on the tank itself. What you are aiming for is to have the tank horizontal (parallel to the divers body) when in trim, but nice and close to the body so it sits in your slipstream. This is what my tank looks like when it's being worn:
Even though the tank is nice and horizontal in this photo, there are a couple of things wrong. Firstly, note that the reg is in one of this octi holders and pointing up. This is not such a good idea, partly because of the chance of free flow but also it's just less tidy. Look how the reg is hanging down, creating drag and a possible entanglement risk. Secondly, the tank is actually hanging down quite a way below the fall line of the body. There's probably a bit of slack where the top clip is on the stage kit, or maybe my shoulder D ring is a little to low down the harness. Either way, bring the tank up a littlle closer to the body would make it more streamlined.