I feel like it is when it is a tall 'show' tank. If it had the dimensions of 30" wide and 24" tall, I would feel better about more polys . Im also considering the fishes at full adult size. When the 16" endli swims around it already makes the tank feel small. The way its going with its growth rate I think there is no issue with it being an 18-20" fish. The other polys are growing slower but I see potential for a bunch of large fish thats all.Your right. Your 300 gallon tank is way too small to add another poly.