What do we use to show frustration in Esperanto texting/tweeting? In English, for example we would use, "ugh" or maybe "sigh"
According to Wiktionary, aĉ is an interjection used to "express dismay at a bad or unfortunate situation". They give the following example: Aĉ, kion mi devas fari nun? -> "Ugh, what am I supposed to do now?"
I have not heard this often, but to me it comes across as less intensely emotional as ve, which is defined by PMEG as "kordoloro aŭ bedaŭrego pro malfeliĉo", so it might be more appropriate in some situations.