I've lost a Barb
 

This morning.... well ok, it was after 11:00 am, anyway, I woke up and went to turn the lights of the tank on, then I did set to feeding the fishes, when I turned the filter off, I saw him, floating upside down. I can't figure the cause, just last night everything was ok, I mean, no rare or erratic movements or very quiet. The water change was 10 days ago (it's a 55g tank), I must confess I haven't the media to make the measurement of the parameters. Just in case, I did a water change (25%) and vaccum the gravel.
I have no live plants, I have no added new fishes, actually the newest ones are the barbs, They are in the tank since 15 days ago.

Without knowing the water levels you may never know the cause of the fish death.

How many fish did you add at once to the tank? You could have caused a mini cycle if your bacteria couldn't handle the load of adding too many fish at once.

And why are you shutting off the filter at times? You shouldn't be shutting it off other than to do maintenance every few months or so (depending on filter size and bio-load in your tank)

Isn't there a local fish store that can help you test the water parameters and diagnose the cause of death of the barb? I think it is important to figure out the cause of death, especially if you didn't see any weird behavior before this.

well, I got a Petsmart, just crossing the border it means about 40 to 60 minutes trip, depends more of the time you spend on the line. The store per se, is at 5 or 6 miles from my home. I've seen two kits there to do the water's meassurement, The "Freshwater Master Kit" from API and the "Quick Dip Complete Water Quality Test" from Jungle. Of course they offers that service free, but I think it's better to get the kit at home.
There is a doubt about the water hardness, how can I modify it?. I'm planning to get the kit and to get what I could need to obtain the adequate levels on nitrates, nitrites, ammonia, pH and hardness, this last one is the one I don't know how to modify.