A 40 fp should fly the plane... not much more. Your down on power so you may need to get as much speed as possible even a shallow dive to complete a nice size loop... that could explain the tail wag and fall out at the top of your loop... of course a tail heavy condition or to much throw could be the problem. The down trim could be caused by a number of thing's... double check the CG and throws... if their in line with the book then look at the elevator with the radio/plane on... put a ruler on top of the horizontal stab and look at the control surface in relation to the ruler... is it level with the ruler? is it hanging down from the ruler or holding it up? the position will tell you exactly what your trim adjustment is doing to the control surface... if it's almost level reset your trim and check it again... you should see the surface is holding the ruler up... correct this at the servo or control horn by moving the servo arm or clevis so the control surface is sitting level with the ruler. If it's down and reseting your trim puts the control surface level then you need the down trim for level flight... it could be wing or horizontal stab incidence is off causing the need for down trim... I would'nt mess with it if that's the case. You could also change your CG to be a bit more nose heavy... it should help with trim BUT it will make the plane fly nose heavy so again unless it's showing sign's of being tail heavy in the air with the CG set correctly(by the book) I wouldn't mess with it... a little down trim on a trainer type plane won't hurt it.

Replace the tank, double check everything is by the book and if at all possible get someone with experience to help you out... the net is great but hand's on is the best help you can get.

John