So a few things to note.
My first install had the anti-cyclone plate on the opposite side to the pump and this didnt work well.
But swapping the tube around with the anti-cyclone plate just before the pump it works miles better.
Secondly, this weekend I moved the tubes around so that you can see a fill port right at the top.
As you can see, I have turned both tubes to have the silver stop cap facing the top.
This way I could fully fill the res.
The first few times I was holding them as upright as possible but this left a lot of air in the tubes.
With the stop ports at the top I can get them almost 100% full, and if I really wanted to I could full them pretty much 100%.
Only thing to note is to have a drain line.
With them being so full there is no much in the way of opening them to drain.
I have a drain line, top right of that pic.
So I just attach a hose and turn on the pumps.
This pushes most of the water out and gives you a lot of room to play with to get the rest out.
All in all running the tube Res horizontally was pretty easy in the end.
But like I say it did take me 2 or 3 mounts to figure out basics.
But now you know