I can't limit myself to one thing - so I'll list two (well actually three)
Sterilite storage shelves and Solo 5oz. cups
At least for the species that I work with (mostly thumbs) - this setup allows me to keep individual tadpoles in a very confined space. 3 racks fit perfectly across a 48" metal rack. I use 6 shelving systems (18 shelves) and each shelf can hold 16 5oz. cups - or 288 tadpoles in a 48"x24"x18" area. Lately I haven't even filled 20% of that capacity, but it is nice to have it.
Also - a
Vornado fan. Great for circulating room air. Works in the winter and summer to keep temperatures more even in the room. While I do take advantage of natural temperature gradients by keeping the species that like the warmth higher up, in the winter the temp near the floor can get a bit cool. This fan does a great job at leveling things off. I think they also make them attached to heaters for those that need them.
Oz
