How did you heat the tubing? I have found it helpful to fill a wide mouth Thermos with boiling water to soften the tubing. Then I make sure to wear gloves to twist the tubing over the fittings. It helps to make sure the fittings and tubing are compatible sizes but I am sure you know that. A quarter inch difference makes it difficult.

I'm also wondering if you're sure you have the right size (and not too thick). I tried to no avail to get some tubing on a barb and realized it SEEMED to be the right size but sure wasnt lol

I used a rubber mallet. It did not take much, I've even pounded the barbed fitting on the ground with the hose above.

HTH

I just posted a similar complaint - one problem is simply incompatibility between different manufacturers, AND between pond products and those made for irrigation and plumbing. Theres a point where it just wont fit - Ive tried to force it only to have the tube split. In that instance I just said to heck with it, and attached the tube directly onto the threaded outflow port and clamping it on with a clamp. It all seems to be trial and error -lf barbs dont fit, try fittings with threads instead of barbed ends - seemed to be a slightly smaller diameter . with clamps I think that may be secure enough.

You can get flex tubing for VERY cheap at home depot and lowes. At lowes you buy it by the foot. I always, always, always use clamps :)

Heat the tubing with a hair dryer, and coat the fitting with liquid soap or detergent. once you get the barbed fitting in use a small socket wrench to tighten the clamp up instead of a screwdriver.

Take the fittings to the store or a piece of the hose you are trying to connect to. Next you have to find the right department at HD or Menards which is the area that displays brass plumbing fittings, there you will find the correct adaptors hose and fittings that fit small pond pumps.

One problem is that fittings are often sized as inside or outside measurements. The difference can cause a bad fit.