5 gallons is also too small to be an effective quarantine for any but the smallest fish.
Also, you mentioned losing a Tang.
Neither your QT nor your dislay are adequate for that fish...stress would eventually have ruined that party if that's not what did in your tang.
Don't get too hung up on "the miracles of QT" and forget the basics: too many fish, or too big of fish in too small of a tank creates stress.
Stress is something you want to remove from new fish, not something you want to give them more of.
All you have to do is look at the very few threads created by the people who advocate QT with such vigor and see that they have problems just like people who don't QT. They lose whole loads of fish, etc.
QT has its place, but it is (clearly) not a miracle, it is not a panacea.
QT is a tool that can be used for either good or for harm just like any other tool.
QT gets promoted so hard that all of the downsides usually get totally overlooked, white washed, glossed over.
Everything about QT is stressful, so it should not be performed lightly.
There are definitely some fish that are healthy enough when purchased that they should not be quarantined, and if they are they end up in worse shape than before.
If you treat every single fish you buy, this is a fact for how you treat fish. You are sometimes taking healthy fish and making them worse.
If you don't do anything to identify these healthy fish upfront (And, perhaps more importantly, you also do nothing to identify the sick fish either.), then it is on you if these formerly healthy fish die in treatment.
The fact that this aspect is not promoted with equal vigor to quarantining is not the fishes fault or my fault. But it will be the truth if you quarantine and medicate every fish that you get without thinking.
If you are in the position where you have to buy your fish sight unseen, and you also happen to be in a position where you have to buy your fish in bulk quantities as opposed to bringing them home one at a time,
then you should probably quarantine every fish that you get because you are getting what amounts to wholesale quality fish. (In case that sounds like a bonus, it's not...you're paying full retail for lesser quality.)
However, if you are in a position to identify each and every individual fish that you plan to bring home and put a new tank, then quarantining and medicating every single one of them would be a foolish second step.
The better idea is to keep shopping for a fish that are healthy enough to bring home, and to get good at identifying the fish that should not come home. Skip meds, maybe even skip observation. If you are in this position, your best bet is to gain plenty of experience in observation while the fish are still at the store and to give each new addition to your tank at least a few weeks (or more!) before the next addition.
Of course not everyone is in that position these days, so good luck if that scenario doesn't apply to you.
You will need luck and a bigger more naturalistic QT tank if that's the road you're heading down.
I would also recommend a microscope. This is a tool that is absolutely requisite in identifying pathogen's before treatment, but which barely merits a mention by the qt guides. I have never understood this — the excuses for it that I have heard are weak.
@Paul B (of all people) has written more about using a scope on fish than anyone else I can name off-hand if you want some posts to look up. if you go outside the hobby, there are actually lots of guides on YouTube and other websites for fish autopsy.
