I would go without the light.
What I usually do is put some old tank water into the backup tank when I do a water change: all that poop and uneaten food is what the bacteria will feed off of. If I don't have as much as I would like, I will put a small pinch of food in with the backup bacteria.Question: What are you feeding the bacteria?
Need ammonia to feed the bacteria that convert it to nitrite. Need nitrite for the bacteria that convert it to nitrate.
You have to keep the cycle going, else the bacteria start dying. Then all you have is dirty dead media.
It all depends on what type of fish are in the tank and how heavy the bioload is:Do I understand that you are keeping a ten gallon with no fish to provide backup media for a ninety with fish? Sounds like trying to run an airplane with a motor scooter engine.
Many people use the small tank as QT, keeping the small filter running on the large tank to keep the bacteria happy and ready to swing over to the small tank.
I've been doing it the way I noted just above for a few years with no issues. The real question I was having was who on here runs theirs with a light vs. those who do so without a light.Not sure it would work to keep the media separate without a constant ammonia source to keep it fed. I guess the only way to know is to use it with fish after a while but it's probably easier to either run a second backup filter on your tank (even a sponge filter would do) and then you can pull it out to instantly cycle a QT when needed (or have a super large filter like a canister that you can pull some media from to cycle a smaller tank without disrupting the main one).