I use six size V's per 100 gallon as a general rule. The heck with diaphragm air pumps! I adore my linear piston air pumps. No problem running a 40 outlet to ~ 3 foot depth 24/7/365 with under 50 watts!
Just a thought, a 50G drum cut in half (lengthwise) would make you the perfect "filter" for the 300 Rubbermaid.
Cheap and effective natural filter.
Basics:
1. drill holes throughout the bottom and sides 4 inches up from the bottom of a 5 gallon bucket.
2. Put pond water pump inside of bucket and plumb the PVC outlet through the lid.
3. Put the bucket in the center of the half drum cut lengthwise.
4. Fill the drum with any number of B.B. friendly media. *****
5. Complete plumbing to pump water from the drum back into the 300G Rubbermaid.
6. Build overflow return from Rubbermaid to the drum "bed filter".
***** This can also include or consist entirely of hydroponic media. Basically this attempts to close the cycle and minimize the need for water changes. The benefits of the potential Oxygen supply to the 300G Rubbermaid would be priceless, the cost however starts to shoot through the roof.
The bio-media solely for B.B. would most likely have some type of cover on the drum to keep out direct sunlight and leaves out of the homemade bed filter. The hydroponic media would obviously require the open surface for the plant growth through the bed media.