You can use Hyaluronic acid (HA) serums after fillers. You should avoid acids after filler injections but in skincare HA is called an 'acid' because of its chemical structure, not because of its pH level. The term 'acid' refers to the presence of carboxylic acid groups in its molecular structure, which are part of its chemical name. However, unlike other acids used in skincare, such as glycolic acid or salicylic acid, hyaluronic acid is not an exfoliant and does not have a low pH. So HA is safe to use after fillers.