Yes that PH
can, will kill a tang - they are used to 8.1-8.4. If I am not mistaken 7.4 is ~ 100x more acidic than 8.4. Get your water parameters in order and DO research the needs of fish before buying - the yellow tang should be in a 4 ft established tank (with excellent water quality), a 29 is far too small for him (55g is considered iffy for these). Slowly raise the PH
over the course of a week, and do some reading or ask questions here about future stocking. More live rock or dry base rock would be welcome also, canisters are not very good for biological filter in sw
, they simply trap waste causing more problems than they fix (if they are not cleaned weekly). A skimmer would be good as well. Sorry for your loss but you need to learn a bit before moving on to more fish...