My convict died on freakin easter

[Sacklerman]

ok so today my like 2 yr old female convict died and i was wondering how it could have died. it didn;t float when it was dead it was well feed so it was kind of fat, i also just did a water change yesterday and it had been 2 weeks probably the longest ive gone in a while i usually do it 1 a week to  week and half. plus i kinda waited a lil longer to put in the dechlorinator. there just seems no reason why it should have died all my other fish are ok i have a 55 gal with 6 giant danios 1 pleco 1 9 inch JD and my jd didnt ever harass it. the convicts body had no marks on it.. actually i knew something was up when it was stay nere the bottom not beinng real active and moving slow and kinda breathing hard.
 one other reason of death would be that its a 2nd gen convict of some cons that i got at a fish store and maybe they were brother and sister that mated and it had bad genetics.  any thought s thanks im kinda depressed knowing i might have killed it. also what new fish should i get??/ thanks

[Stormbringer]

agree with Gerry , test the water fore adding anything else

[Sacklerman]

thanks guys i didnt know a fish could die of being fat but she always ate the flakes from the danios too. also what do i test for ph hardness or what thissss suxxxxxxx

[Sacklerman]

sorry you guys dont suck  this situation

[Stormbringer]

ammonia nitrite and nitrates along with ph mate 

[mrbigfish]

sory for your loss, you'll get it figured out.