I didn't say they are F. acus, I said they are in the Farlowella acus group.
seriouslyfish.com is ok source but I prefer to read systematics and taxonomy if possible
COMPARISONS.
This specics closely resembles F. vittata and is distinguished from it by having in the adult male a shorter snout-mouth length relative to interorbital width, and breeding odontodes on the preorbital ridge. The ratio of snout-mouth to interorbital width for adult male F. acus is 1.3-1.8, and for adult male F. vittata is 1.9-3.1. Pigmented stripes in the caudal fin are more obscured by clear ocelli in F. acus.
So imagine F. acus with rostrum ratio 1.8 and F. vittata with rostrum 1.9. Almost identical to naked eye