I've been able to re-use Dektol if you pour a lightly used working solution into a bottle, squeeze the air out, and use it the next day. Anything longer than that, or if it's in the tray very long, and it turns black. It still has some activity but it's clearly not very good. Horrible tray life is my biggest complaint about it (it also stains much worse than LPD - metol versus phenidone I think.)