I've got a 2.5 gallon tank in the kitchen window. Planted with gravel, rocks, driftwood, a couple endler/guppy hybrids, a couple ghost shrimp and saganesque numbers of baby rams horn snails. No filter, no air pump, sunlight only, 30-40% wc every 10 days or so, top off with 60/40 tap-distilled mixture as needed. My cats use it as their primary water source (followed by the betta tank and my algae farm bowl and only then their freshly filled daily water bowl). Been that way for about a year now. No losses, no illness (fish, shrimp or cats), no problems. Well, other than having to clean cat hair off the surface now and then.