Is my inline diffuser working properly?

Story:

I got my co2 goodies and lily pipes last week. I rescaped my 20 gallon by taking out the stem plants and planting:

- HC patches to form a carpet
- P. Helferi (plants weren't doing well and replanted them hoping they'd recover)
- S. Repens

I used 2 Jardli glass lily pipes and hooked up the co2 with a Jardli glass inline diffuser. My filter is an Eheim 2211 classic and since the outlet pipe is 9/12mm, I had to attach a hose adapter so I could fit the 12/16mm lily pipe and inline diffuser. I dosed an all-in-one liquid fert from 2hr aquarist and added some Neo-Soil tabs. I'm using a Finnex 24/7 planted LED set with around 4-6hrs at max and mornings mainly reds and evenings mainly blues.

The first 2 days, I noticed pearling and everything looking nice. Diatoms have since taken over and everything looks brown and dying. I've heard this is normal and should just wait it out, but the leaves on my S. Repens look mishapen and crinkled.

I'm trying to consider what the problem is and so far I've narrowed it down to:

Flow-rate: As said, my filter uses a 9/12mm hose on the outflow and I've had to use a 12/16mm adapter and hose to make my lily pipe and diffuser fit. I think this must have significantly reduced flow rate as I can barely feel any movement unless I put my finger inside the lily pipe. The water above the lily pipe looks very still too.

Diffuser: After hooking up the diffuser, I realised the bubbles seemed too large. After some online searching, I was told I had to soak the membrane for 24hrs prior to use. I did that by turning off the co2 for 24hrs and just letting the filter run. Afterwords the bubbles still seemed large and it looks like the bubbles are just flowing to the surface from the lily pipe. The drop checker reads green, but the state of the plants says otherwise.

Acclimation: The plants might have failed to adapt when transferring them from the aquarium store to my tank. If that's the case, not sure what I can do about this.

Are these bubbles too large?
https://vimeo.com/user102343291/review/356789848/82a699668b

Any thoughts?
Thanks