freshwater aquarium troubleshooting

Question
I have run out of ideas why my 10 gallon freshwater tank is getting cloudy,
causing the fish to die. I had 4 danios and 4 platties, now I am down to 2
platties. As regluar maintenance I change 25% of the water every 2 or 3 weeks
and the water will be clear for a few days then turns cloudy.  The strip tests
show that all levels, with the exception of hardness are normal. I took water
to the fish store (Petsmart) and they could find nothing wrong with it.  I
wasn't actively vacuuming my gravel, so bought a new siphon vacuum, and
yesterday gave the gravel a thorough vacuuming, put a new filter in the bio-
wheel  and the water looked great for 24 hours. Now it's all cloudy again, and
it looks like there's smoke moving around in it.  I can't tell if it is particulate
matter or tiny bubbles.  I know I probably threw off the bacteria balance by
vaccuming so much, but am at a loss for other solutions.  The water is hard
here, and I only have one small live plant.  Any ideas?  Thanks!!

Answer
Hi Christine
How long has the tank been set up?  First suggestion I would make, buy the dropper test kits.  I've always heard the dip stick ones weren't that accurate, and just found that out myself a few weeks ago.  I know Petsmart uses them as well.  It's expensive, I just paid 30.00 for my test kit, an API brand with ammonia, nitrites, nitrates, ph, and high ph, but it's so much better.  Now, your test kit and Petsmart's show normal, but you don't want any ammonia or nitrites in the tank, that's toxic to fish and will kill them.  Some people think a small amount of those are fine, but they're not.  Nitrates should be ideally under 20 ppm.  

Now what color is the cloudy water?  Green is an algae bloom, shouldn't really affect the fish.  More of an "eyesore" for the humans :)  White or grey is a bacterial bloom.  That's common for newly set up tanks, usually clears on it's own in a week or so.  It's basically the bacteria forming.  

You're tank was a bit overstocked, depending on what kind of danios.  I have some of those small zebra danios, they won't put too much strain on the bioload, but if they're the larger giant danio varieities, that may have been too much.  

For your tank, assuming it's done cycling, get on a schedule of changing 25% of the water once a week instead of every 2 or 3.  Don't change your filter pad at every cleaning, or even once a month.  When you do a water change, take the filter pad out and scrub it with your hand in the bucket of used tank water and reuse it for at least 3 months  or so.  There's bacteria on the filter pad, the biowheel, the gravel, decorations.  Probably wasn't a good idea to replace the filter pad and do the good gravel cleaning-if it's a newer tank especially.  I have 2 filters on almost all my tanks, so I can get away with that :)  On yours, if you intend to change out the filter pad, just do a light gravel cleaning, just skim the top of the gravel.  I've read where some people only vacuum out 1 side of the tank at a time, then the other side at the next cleaning.  Also, you mention that you weren't cleaning out the gravel very well before, and now you are.  That may be what's causing it as well, stirring up all that food and poop from underneath the gravel will make it cloudy.   

Just let your tank be it'll clear out, keep an eye on those water parameters too.  Sounds like the bacteria maybe did get disrupted somehow, and with 2 fish in there, you may need to do small daily water changes for them to survive.  But otherwise, just follow my advice and do the weekly changes.  And don't add any new fish for awhile.  Wait till it gets straightened out.

Hope that helps and good luck!!

Christy