Typically the float step should be at least 1/2" behind the normal CG point . Some planes require it to be 1" behind. Other planes fly better with the CG on the step. Usually when the step is forward of the CG the water handling is not so good.
Next you have to check the incidence of the wing related to the top of the floats. The wing should have about 2 degrees positive incidence and you adjust the float brackets to achieve that
Usually a plane becomes nose heavy when the proper size floats are installed. It sounds like the floats you have may be too short and/or the atep is ahead of the CG.
With the step set behind the CG point , the tips of the floats should extend a bit beyond the prop nut or spinner
To determine if you have the right length floats you first need to measure the length of the fuselage from the front of the thrust washer of the engine to the end of the vertical stab (not the rudder). Then take 75% of that length and that should be the length of the floats you will need.
Try and keep the floats as light as you can and you will probably need a water rudder on one of the floats if not both. The rudders can be tied into the planes existing linkages or a seperate servo can be used and "Y''ed" into the rudder servo.
The Telemaster will make a nice float plane.