Assuming you're using a diffuser (which is a bit small for a 55g, and really small for a 125g, but something is better than nothing). Also assuming you have a rig that includes a mounted needle valve and a bubble counter.
Simple and cheap-ish suggestion:
inline bubble counter
1 x 1/8" NPT tee, 1M 2F ports
1 x Fabco-Air NV-55 needle valve ($23.05 + shipping)
2x 10-32 to 1/8" barb
1x 1/8" NPT to 1/8" barb
Remove the current needle valve.
Attach the tee to the port the needle valve occupied.
Replace the needle valve at one end of the tee.
Attach the 1/8" barb to the other end.
Attach the two 10-32 to 1/8" barbs to the NV-55.
Run tubing to the NV-55, and from the NV-55 to the diffuser.
The NV-55 is inline, so no need to worry about mounting it. Alternatively, you can buy the NV-55-18 with 1/8" NPT ports and mount it instead of running it inline, but it's an extra $14 or so. The NV-55 is a decent needle valve, and you'll have a jumpstart for when you upgrade your system (which you'll eventually do).