There are a lot of great suggestions bouncing around on solving your issue, but I'm wondering if we could get some more information on your system...
- Are you doing any other types of nutrient exports like carbon dosing, Zeovit, skimmer, water changes, etc?
- In your Ruby sump, is the Chaeto in the first chamber with the filter socks?
- Does the Chaeto tumble or float on the surface? If floating does it die back after it gets really thick?
- What is the turnover of the return pump in regards to your total water volume?
I went through a similar issue and eventually I was able to solve my problem, but not before trying carbon dosing and an ATS. Both methods worked, but I really wanted the benefits of macroalgae in a refugium so finally gave the Chaeto a third try after months and reading everything I could find on growing Chaeto successfully. Looking back I think my issue was a combination of things like a lack of elements already mentioned and low flow across/around the algae bed.
For other options - As mentioned by others, carbon dosing and an ATS will work great, all have pros and cons. If you do look into an ATS I would consider the Turbo Aquatics L2 or L4 that Bud makes. I looked at all of them last year and I feel he has the best design with a well-documented track record of happy customers. I'm not affiliated with him at all, just have experience reading on all the various options available in 2017 and first-hand success with the L2 I purchased last year. The only reason I'm not using it now is I feel the refugium is the lowest maintenance option assuming you can keep the macroalgae alive.

I did have both running for a while, but my tank was too clean so I took it offline and packed it away for another tank down the road later this year that will not have room for a properly sized refugium.