PokerStars - $0.02 NL FAST (6 max) - Holdem - 6 players

BTN: 73 BB
SB: 185.5 BB
BB: 147 BB
Hero (UTG): 108 BB
MP: 101.5 BB
CO: 100 BB

SB posts SB 0.5 BB, BB posts BB 1 BB

Pre Flop: (pot: 1.5 BB) Hero has 9 T

Hero raises to 3 BB, fold, fold, BTN raises to 6 BB, SB calls 5.5 BB, fold, Hero calls 3 BB

Flop: (19 BB, 3 players) 9 K 5
SB checks, Hero checks, BTN checks

Turn: (19 BB, 3 players) A
SB checks, Hero checks, BTN bets 5 BB, fold, Hero calls 5 BB

River: (29 BB, 2 players) 2
Hero bets 9.5 BB, BTN calls 9.5 BB

Hero shows 9 T (Flush, Ace High)

Small raise from a player on microstakes often means he has a decent hand but doesn't want to play big pot, PPs, broadways and even some garbage hands like 63s. But generally fold is too easy on Zoom so they show up with a good hand.

On this flop I probably fold to a bigger bet. Pool plays rather tight multiway. BTN checks, I assume he doesn't have Kx or set. On the turn BTN bets his Ax, hands like QJ and flushdraw. I have right odds to call. Now on the river I'm not sure whether I should bet or not. V probably doesn't have much two pair. I think with a small bet I don't lose any value.

I'd go for the check-raise otr, but if you're gonna donk do it bigger; villain is a fish you don't need to bet small like that.

The preflop call is ok against x2 3bet and caller. Flop and turn are ok too.

Agree with @JamAdebayo, if you decide to bet, then bet with a large size 3/4-PSB.
If fish has an Ace, he doesn't care what size to call.