Our toilet tank thingie (Fluidmaster brand "vertical assembly thing"...comes with a flapper located adjacent to it in the tank) is "hissing softly" and "bubbling slightly" at the very bottom of this vertical assembly where it meets the bottom of the tank. It makes this sound and bubbles every 16 - 20 seconds. This has developed concurrently with a sharp rise in our water bill. Funds are tight and I really don't want to call a plumber if I can help it. Is there only one size/type of Fluidmaster item of this sort? Is this a relatively easy fix? What do I need to tell the person down at Lowes/Home Depot in order to obtain the right parts to make this repair? Thanks...

For standard toilets, the Fluidmaster is real standard. And it's easy to install. You might be having a flapper issue. I'd shut the water off at the toilet valve and remove the flapper and take it with you. They can vary. Get one similar, and clean the 'seat' that the flapper sits on. Not expensive parts.

A plumber would charge between $80 - $100 for this type of repair and that includes the parts. Compare the price to the water bill, it just may be worth calling.

there are actually two sizes that I know of of this filler assembly. the fact that it's periodically tripping on and then stopping without any external leaks tells me the flapper is leaking. the plastic they're made of gets stiff and creepy over time as chlorine in the water supply attacks the plastic.

you need a new flapper, and if you spend more than $6 for it, we will all come out and "nyaahhhh! nyaaahhh!" you.

seriously, it's a quick fix and a cheap part. the only two things to watch for are not leaving enough slack in the pull chain/zip tie end (there should be a little slack, maybe 10 degrees of rotation in the flush lever) and not hooking the ears down fully on the vent pipe.

I would also use a damp towel to run around the seal the flapper drops onto. this is one place crusty plastic from the flappers likes to collect, which itself can cause slow leakage out of the tank.

Got the filler assembly ($8), installed it, problem persisted, got a flapper ($4), installed it, fiddled with chain, cussed, fiddled with chain more, problem fixed!!

yeah, the fiddled with chain part is something you have to discover for yourself. too much slack, it ties itself in knots and holds the flapper off the seal. too little, you build in a leak. has to be about "two Goldilocks ticks" slack.