4 year old 1/4 hp submersible pump began tripping GFCI. Replaced motor-still tripping. I replaced 15 amp GFCI with 20 amp GFCI - still tripping. I filled a plastic trash can with rain water and plastic bucket with pond water - both pumps trip a (different) 15 amp GFCI when submerged in the pond water bucket and both run fine when placed in the rain water can. Pond water test strips appear to show acceptable levels. Can water chemical affect GFCI performance? I recently began adding salt and cyanuric acid because of low test strip levels.