Execute and Testforblock

I was running a fill clock and testing to find if the player is standing on soulsand like this: /execute @a ~ ~ ~ /testforblock ~ ~-1 ~ soul_sand getting a comparator output and taking it to an /effect @a 1 1 1 and /setblock blah blah blah that reset the execute command block and set its true value to false. however the comparator was rarely giving an output. please fix so i can do a tutorial on this. thx. p.s. please make a void world superflat preset (desert biome)