Today I'm noticing smelly water in the 37 again. We added Sebastian on friday night. I tested the amonia and the nitirite today, didn't have time for all of the api tests. Nitrite was fine and amonia was just a little darker. (Seems to always be like this.)

What on earth is causing this? Could it be a mini cycle starting? I'm thinking I might need to do a water change tonight, but should it be a 75% or something like 30%?

Could a loaded filter cause a smell like this? One of the filter pads is due for a change. We alternate changing them.

I can do a 50%.

Don't ever change all of the filter media at once!! Now you're for sure forcing the tank into a mini cycle
Rinse the filter pads in tank water (that you obviously remove from the tank first) during water changes, once a week or every two weeks. It's really important that you take care of the filter media or you'll have problems with ammonia all the time. When you notice the filter media getting sort of softer and more "used" looking you should change only half the filter media then and the other half after a month.

Sure, hopefully it won't be very noticable