hello everyone, i write to you with a heavy heart tonight. allow me to explain...ugh. maybe i'm being too sensitive about this, but i feel absolutely horrible over what has happened. surely as fellow pet owners you'll be able to understand my upset. here's my timeline. bear with me please. i have been following the advice of pet store employees. it never occurred to me that perhaps i should go online and search for chat rooms dedicated to fish keeping. i just...i don't know. i didn't give it much thought and just asked the employees for help time and time again. i think they mean well...but...yeah. here it goes. june 1st - bought 25 gallon tank, set up with aquaclear filter, heater, airstone, gravel, several ornaments/artificial plants. it sat fishless for three weeks, filled with water and filter and heater running during that time frame. june 21st - bought three zebra danios for a fish in cycle. on the advice of pet store employee, a young girl, who i truly believe meant well. added nutrafin cycle, which is a three day cycling treatment. online articles give mixed reviews. bought an api master kit. began testing every 24-48 hours, while dosing the tank with prime every 24-48 hours, as well. did 25% water changes each week, like clockwork. didn't miss any. very diligent about being on time. vacuumed about 1/2 the gravel each time. things were going marvelously. not a single sign of stress in the fish. active, eating well, socializing. my ammonia climbed to 0.25, nitrite stayed at 0 (perhaps i missed the spike...?), and nitrates climbed to 10. my ammonia held firm at 0.25. i read that 0 is ideal, but that some tests will show 0.25 even when there is barely no ammonia present. july 1st - bought two bloodfin tetras, and one powder blue dwarf gourami. things went marvelous with them, as well. not a single sign of stress. no fighting among fish. everyone ate a variety of foods with no problems (flakes, pellets, vegetable matter, and blood worms). i continued testing every 24-48 hours, and dosing with prime at the same time. my levels remained strong at 0.25/0/10. i continued with the weekly 25% water changes. fastforward to july 28. bought two zebra danios, and two albino corydoras. please keep in mind as i'm saying all of this that i wasn't up to speed on what it meant to form a shoal of fish. the employees told me they would be okay with two or three in a group. again, it didn't occur to me to research this myself. i took their word for it. everything was perfectly fine and dandy until i added these fish. within 12 hours, the two new zebra danios were dead. within the next 12 hours, the corys were dead. within the next 12 hours, my three previous existing zebra danios were dead. tonight - one tetra just died. one tetra and my dwarf gourami remain...but not for long. i can guarantee you they are dying by the behavior they are exhibiting, which is unlike their norms i've come to know over the weeks. so, every fish basically began exhibiting the same signs of impending death as soon as i put the new fish in the tank on the 27th. floating in space, not putting much effort into swimming, listless. no aggression. they all died in the exact same manner - they would suddenly flip over, float along on their backs while attempting to swim, attempting to flip back over with no success, and then die while sinking to the bottom slowly. so, my assumptions... first off, bear in mind that i've tested my water parameters several times in the last few days and there has not been either an ammonia or nitrite spike since adding the newbies. not a single inch of movement with either values. the water remained bright and clear. therefore i am not quick to blame it on an overloaded bio system. the two new danios and the two corys i added on the 27th...perhaps one of them was diseased. please bear in mind also that i didn't know of such a thing as a quarantine tank. i just read about it within the last hour of sitting here. i figure one of these fish was the culprit because the rest of the fish were living very active, content lives leading up to the newbie additions. i didn't see any outward signs of disease. no strange colours, lesions, no strange bowel movements, no bleeding, etc. they looked normal, in my eyes, to the best of my knowledge. so...if you've read this far...thank you SO much. i feel like a lost soul here...no idea who the heck to turn to for advice. all i know is that i do want to keep fish as a hobby and i care about them very much and am willing to do whatever it takes to become a responsible fish keeper. my questions...what on earth do i do now? so many questions. if anyone could try their best to answer them in order, it would be so greatly appreciated so that i don't get confused. well, any moreso than i am right now. ...i don't even know where to start... do i empty out the tank? remove the water, turn off the filter, remove the gravel, etc? can i reuse this gravel and my ornaments/artificial plants in a future tank, or will they have to be thrown out in case there are germs latched onto them? i really hope i can continue using them all because it cost me a lot of money to decorate my tank. assuming i can reuse my tank and accessories, how do i properly sterilize them so any future fish will not pick up any germs? assuming that my tank came down with a disease in recent days. god...please help me...i'm up against a wall here. no idea who to turn to. heartbroken, and my children are sad about it, too. i feel like i've let the kids down by killing their pets. i'm getting ready to leave on vacation for a large chunk of august, so i won't be able to do much for at least 3 weeks.