How do I create a transparent liquid for ferrofluid display?
There's a really good video on YouTube by Nilered/Nileblue on ferrofluids. In it he suspends magnitite with oleic acid (main component of olive oil) as a surfactant and uses Kerosene (Paraffin Oil) as a carrier.
There's a PTFE 'dry lubricant' sold under the WD-40 brand name that might be able to prevent the glass from staining as it creates a thin coat of PTFE on the glass which is highly inert. Just spray the piece of glass with the PTFE Spray and wait for it to dry (~7mins). [The PTFE spray has a myriad of other uses too, you can put it on any surface you want to have low friction, to be lubricated, an unreactive surface, dust repellant surface, etc].
This might be a way around the staining issue you are getting. Or at least point you in the right direction. Good luck!