I dont think Ive seen 500s around for a while... Wont need it for a few months now though.
I know what you are saying about the likelihood of cross contamination I just cant think of any equipment I would use on both tanks. one tank is 160L the other is 1600. Totally different size nets and syphons... Might have hitched a ride on my arm perhaps though Im pretty careful to wash my arms before sticking them in another tank... Only possible explanation I suppose.