You don't need another tank. The one you have will work perfectly well for smaller batches. There is no need to completely fill the tank with chemistry. Only enough to cover the reel(s) containing film is required, and those quantities should be indicated on the bottom of the tank. There is no need to invert the tank to agitate either, though you can do that if you like. The agitation stick works perfectly well provided you use it vigorously enough. There is also no need to use more reels than necessary. Tightly wrap a rubber band around the central spindle above the reel and the reel won't creep up the spindle when you invert the tank.