Fishy you have a 2 foot , 25 gallon tank with a tang and a butterfly in it. Much much too small a tank for these type of fish.
For a tank you size 3 small fish would be enough

Sorry to hear about the fish losses :( Never a fun thing to come home too.

Overstocking isnt likely the problem , all depends on equipment .

First off I definitely am sorry to hear about the losses. It's certainly a terrible thing to lose any fish, let alone that many in such a short period of time.

But I have to disagree with this statement. I'm not saying it was the cause of this situation, but overstocking a small tank can definitely lead to this outcome.

With the relatively low surface area in smaller tanks, all it takes is the temp to go up and then the oxygen saturation goes down with it, and then it's all downhill from there.

Other than the blenny, all the fish are quite active and require more oxygen rich water than a perching or substrate dwelling fish would. I have no idea what kind of equipment he was running, but for the most part no small tank has an excessive amount of equipment running in it. Cost and space are prohibitive. Which is why the margin for error with them is usually much smaller than a larger tank.