We have a fluidmaster 200a fill value and the water is just trickling into the tank. should we replace the whole unit?

Toilet tank won't fill up or is slow to fill. The most common reasons for a valve to slow down or no longer fill the tank after the flush are debris issues and the length of time a valve has been in use. ... Valves in use for a period of 7 years or more can have their working parts start to wear out. 1.
https://www.fluidmaster.com/toilet-problems/remove-fill-valve-cap-replace-seal-toilet-wont-fill-slowly-filling-tank/

First shut water off at stop than flush all the water out of the tank next take a sponge and get all the water out of tank next unsrew the supply line from the valve under tank than move out of the way next unsrew the nut on the valve of fluidmaster than pull the fluidmaster out of tank stick the new fluidmaster in tighten nut srew the supply line back on turn on stop and let the tank fill up thats it if the fluidmaster dont shut off press the handle let some water out and adjust the fluidmaster screw Tip the water should stop a 1/4 inch below the flush valve tube in center of toilet

Take off the cover and look at the mechanism when you flush. It's possible the set screw that keeps the flush rod in place has just loosened. If not then the easiest way to deal with this is to replace the Toilet Refill Valve. Turn the water line at the base of the toilet to off. Flush until most of the water is out of the tank, the go to Home Depot and buy this:

Fluidmaster Toilet Tank Fill Valve 200AX The Home Depot

Unscrew the old one and screw in the new one.

Plastic pieces inthere that give up after a while. I would go ahead and replace the assembly. But try this first,disassemble the top of the valve assembly and remove any debris. It could be the problem. There are rubber and

Install the FloLogic System, a whole-house leak detection and water shut off valve. You will sleep better knowing your home is protected 24/7

ok,, if you can lift the arm and it shuts off its just aligned wrong, shut off the water, reach down to the lower half of the fill tube,, you will see a little slide clamp, that slides off, and you can adjust the unit up or down to set the amount of water the tank allows to fill and turn off , also, if the over flow ( the small black tube) that goes down the tube that holds the flapper ball, is too far down inside of that tube, it will create a siphon so make sure thats attached to the metal clip that comes with the unit. If the water doesn't shut off when you lift the arm on the float bar,, you can turn the top of the unit 1/8 of a turn, and remove the top, check for any debris inside of that , you can invert a cup and hold it over the top long enough to shoot a short blast of water to clear the feed out. reassemble, and turn the water back on,, you may have to readjust the slide tube to the desired water height

Replace the top cap of your fill valve, or just the fluidmaster seal disc. Here is a link to the Fluidmaster fill-valve Instructions.

SHUT OFF WATER.REMOVE WATER FROM TANK.UNSCREW SUPPLY LINE NUT TO FILL VALVE, (UNDERNEATH TANK). UNSCREW FILL VALVE NUT FROM FILL VALVE, (UNDERNEATH TANK). REMOVE OLD FILL VALVE. INSTALL NEW VALVE IN REVERSE ORDER.

it's probably your fill valve has gone kaput! try replacing that. turn the water off disconnect the water suppily line and there should be one nut under the toilet tank and then the whole valve should pop right out and you can pick up a new one . fluidmaster makes the best one! and just follow their instructions on installing it.

Some Toto fill valves have a plastic debris screen located at the inlet connection. Turn off the supply valve and remove the supply connection at the bottom of the tank. Use a mirror to inspect the screen for debris.
I agree w/ Smithbrother, the Fluidmaster fill valves are cheap and work well. The only thing they ever need is an occasional seal for $1.50 and 5 minutes to replace.