Rendering a Koi pond

I consider myself a reasonable DIY'er but would like to get a straight answer on the following question.I have left over from the building of the pond (which is 7ftx7ftx6ft deep using Dense concrete blocks) about 1/3 of a jumbo bag of building sand. Can i use this for the render using it in a 3 sand to 1 cement ratio? I will be adding fibres to the render and the final finish will be fibreglass. I am getting conflicting answers from websites, some saying use building sand and some saying use sharp sand. Thanks in advance for your time in replying.

The guy who rendered my pond used a mix of fine sharp and building sand around 3:1 as it made the mix easier to work. You may get away with just building sand on the base but it tends to fall off the walls.

Jason

Thanks for your reply, that was my kinda thought on it.