Diseased Fish?

Question

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What exactly are the water parameters reading-ammonia, nitrites, & nitrates?  None to almost zero for each.

Have you noticed the fish rubbing, scratching, flashing on objects in the tank?  They do not appear to be doing that.

Or darting around the tank? The guppies always dart around as if playing.

Did you notice if they were gasping at the surface of the water at any time?  No gasping at surface.

Or hanging out at the top a lot, or around the filter output area? No.

How do the neons look, is their color still deep blue/red?
The scales on the gourami's head, does it look like a white or grey film on it or on the body?  

The gourami in question died during the night.  Upon inspection when I removed it, I saw no filmy substances, but the all its scales on its head and part of its spine were gone.

However, I noticed some white on the top of the fancy guppy's heads or body (they are yellow and indigo), but am not sure if this is just their normal coloring from the light.  The neons all seem fine and brightly colored.

And how's it's mouth look any red areas or white stringy stuff on the mouth/body?   None.

The ones crashed at the bottom of the tank, are their gills moving fast?  Well, the two that were at the bottom of the tank didn't stay there very long.  They are swimming around again like normal.  I did not notice if their gills were moving fast.

And are their fins moving or are they clamped at their sides?  Fins are moving.

When you disturb them or tap on the glass do they swim ok, or do they have a hard time swimming?  The gourami had a hard time swimming, as though his back was broken or something as he could only move turn in one direction.  As for the others, they swim about when I tap on glass without problem and come to the surface whenever they see me.

Are you using those blue fiber filter pads that come with the carbon in them?  Yes.

What's in your tank, gravel decorations, etc.?  And how long's the tank been up and running?  Gravel, two fake plants and two decorations.  Tank is 9-10 weeks old.

When's the last time a new fish was added?  Two weeks ago we added a balloon molly.  We did notice that this molly had a tendency to chase after the gourami that died.  Also, since then, we had a swordtail, three guppies and the gourami die.  

I also can't tell if there is "pop eye" happening or not.  Sometimes the eyes look like they are a little protruded but again, I don't know if that's their normal state or if it's pop eye syndrome.

Thank you so much for your help!

Answer
Hi
Well, as long as your ammonia and nitrite are reading
0 ppm, and nitrates under 20 ppm, that's ideal.  You said the levels were 0 or almost 0, so I'm guessing it's the nitrates that are showing a small amount.  That's fine, and since it's a newer set up tank, it probably just finished cycling.  

See if the increased water changes help, and get back with me if you start to see any symptoms showing up.  Also hold off for awhile on adding any new fish to the tank.  Give it a few weeks(2-3) to make sure there's no problems going on.  
Good luck, and hope that helped!!

Christy

You're fish don't seem to be showing any signs of a parasite infection, or bacterial infection.  I'm thinking they're just a casualty of the cycling process in the tank.  Being a newer set up tank, it won't have as much beneficial bacteria as a more established one.  Throwing out the filter once a month, may have caused the cycle to extend a bit longer, or even have a mini cycle to occur.   

Community fish and especially livebearers in my opinion, just don't seem to be as hearty as say, cichlids.  A guppy bought at the store, is most likely an older one-so you can get the bright fancy colors.  I'd have to say the average lifespan is about 2-3 years old, tops.  Longest guppy I've had was 6 months.  Platies I"ve had 1 that lived about 3 years, but the rest all went within a years time.  It could just be where you're purchasing the fish from as well.  If you're in the US, there's a well known W store that sometimes sell fish.  A very good majority of those stores, their fish are kept in awful conditions.  There's lots of petstores like that as well.  

I'm going to suggest upping the water changes to once a week(7 days), and change about 25% of the water, doing a thorough gravel vac as well.  Rinse off the filter as I stated, and discard it after about 3 months.  Just keep an eye on them, and look for signs like the questions I asked ie: darting around the tank, rubbing on objects, etc.  

As for the gourami, it's hard to say.  It very well could've been an injury that got infected, or he could've  scratched it on the decoration which then got infected.  One thing with gouramis, now all fish have their own personality so this isn't a sted fast rule.  I've always heard not to keep more then 1 gourami(think you said you had 2).  They're labrynth fish like bettas, and I've never had luck with more then 1 gourami.  Whenever I've put 2 in a tank, one always ended up dead within a week.  I never saw them fighting either, and of course after the last one I had died, that's when I started reading not to put more then 1 in a tank.  

The molly may have been picking on the gourami, because she knew it was sick.  They tend to do that sometimes.  As for the popeye, keep an eye on it.  I've seen some balloon mollies with larger eyes, some not.  If the eyes start to look larger, or if she starts swelling up or looking like she's pineconing-the fins sticking out, then it's dropsy which usually goes along with popeye.