I'm new here, and I have a question.

[Subatomicparticles]

looking back through the whole post it's possible to be ammonia poisoning the symptoms are there,
very active than suddenly not so active ,not eating , hiding
rather than depending on when you get to pet store to have water tested
I would suggest is getting yourself a good test kit
a lot of times things can be caught early if you test on a normal basis and keeping a log , a log can become a useful tool as it has a history and if you keep it up to date you can see things going down hill and correct them before something like this happens again
sorry bout your lose as I know these fish are not cheap
another thing I would suggest would be more frequent routine water changes
it will improve the overall health of your tank and make all the inhabitants a lot happier
also research everything about each and every fish so you know how to care for them what,they eat , habits , compatability
ect , same with any inverts ,and corals ect
the more you know about your whole tank will make it that much easier for you
when a problem arises
remember behind every happy tank is the happy owner
you can have pride when showing your tank and knowing you can answer questions if anyone ever asks
another thing to remember a well maintained tank
will make it that much easier when you preform your weekly maintenance
saving you time and money and giving you more time to enjoy your tank

no one here will steer you wrong as we all want that perfect tank
and let me tell you you are well on your way

Thanks, I'm trying to do everything right, although I did feel like I wasn't changing the water enough. Other than that I've been doing everything the pet store told me to do.

 

[jeffaquarius]

I posted everything you asked for, and you then later say I didn't post enough.......right.

A test kit is a must for an aquarist and 2 people already suggested that. Without any info of water parameter it is impossible for anyone to accurately analyze a problem except guess. There are only 2 things possible why your fish died. Poisoning due to toxic water or being sick that is usually caused by parasite. They both have similar symptoms you mentioned but the later has visual indications on its body.