Unexplained fish dying in 10 gallon tank

Question
    Hi there -- hopefully you can help me (and my 10-yr old daughter)with our fish tank problem. Sorry for the length of this, but I wanted to give as much info as possible. The situation is this: we have a 10-gallon freshwater tank that we set up 16 months ago.  We cycled it with 2 mollies, one of which is still alive and well today. Shortly after we got the tank the female Molly (Puddles) gave birth to lots of fry (on day 4), so she was apparently pregnant when we bought her.  We saved the fry that the male molly didn't gobble up and raised about 10 of them successfully for about 4-5 months, but then started to die one by one, for no apparent reason, over the next 4-5 months.  The last of the fry died about 5 months ago, so he almost made it a year.  When we were down to about 4-5 babies plus Puddles, we added 5 neon tetras, of which only one remains (now living around 8-9 months in our tank).  About 6-7 weeks ago, after things seemed pretty stable, and we were down to the original female dalmation molly and two tetras, we added 2 more mollies and a platy.  The platy died after a few days, and it seemed as though he must have been sick when we got him.  We "returned" the platy to PetCo (the source of all our fish so far), and brought home a pleco and a guppy.  Then  about 3 weeks ago one of the mollies died, and just a few days ago the second new molly died, followed by one of the two remaining tetras (last week), and then tonight the pleco died.  All of these fish seemed healthy until either right before or shortly before they died.  They seem to just get suddenly weak and crash to the bottom, and die a day or so later.  The last Molly to die ate (somehow mustered the strength to get up to the top to feed, but then crashed back down to the bottom) right up until he died. The pleco death tonight was a complete surprise when my daughter went up to bed tonight -- he was pretty active for a pleco and seemed fine yesterday.  I'm wondering if he starved....because we don't really have any visible algae, although he seemed to be eating most of the time.
    None of the fish ever displayed any signs of disease (ich, fungus, fin rot, bloating, dropsy, gill disease, etc.).  In fact, we had problems with fungus and fin rot during the first year of the tank that we successfully treated, and those problems have disappeared.
  Each time I take a sample of our water to PetCo for testing, they say it's "fine."  The water in our area is extremely hard, but according to local fish people I've asked, they say that Molly's and other live bearers typically  adjust to the hardness in this area.  Also -- our water is crystal clear and we've had no algae problems.  Just white calcium build-up on the filter, etc.  The temperature is a little high, especially now that the house is warmer -- it's around 78 to 80 degrees with the thermostat on the heater turned as low as it goes.  We're not fabulous with water changes (i.e. not weekly) but do about a 20% water change and replace the filter media every 3 weeks.  We add a little bit of aquarium salt each water change.  We experimented with stopping the salt and trying to decrease the hardness of our water by using some bottled water, but during that "experiment," the fish developed fin rot.  So we went back to our old method.   Here are today's numbers according to my Jungle Labs test kit: ph - between 7.2 and 7.8; alkalinity - "ideal" (120); zero chlorine, hardness = 150 (maybe a little higher); zero nitrites, and nitrates = 40 (upper end of "safe" zone).  I seem to get these exact same readings each and every time I check the water, which I do whenever the fish seem not right (or have died).  There is no ammonia either.
    So I'm hoping you can explain why the fish are dying at odd intervals with no warning or obvious disease, despite the resilience of Puddles, the original female Molly, and this one little tetra who's made it for much longer than the other 4...  I'm hoping to solve the problem before poor old Puddles succumbs.  Could it be unhealthy fish from PetCo?  Could it be the hardness?  Could it be that we really need to do weekly changes?  By the way, we use dried flake food (Tetra brand), and I try not to feed them too much -- only a few flakes per fish.  We have plastic plants, substrate, a whisper filter, and a bubbler that sometimes works, sometimes doesn't.  The tank gets very little sunlight so algae has not been a problem, and, we've never added a bunch of fish at once.  One more thing -- we did have an ammonia problem during the first 6 weeks after the fry were born (and due to overfeeding), which I think is what killed the original male molly.  But since then we have been very careful not to overfeed or overstock, and monitor the ammonia carefully each time we add new fish.  Thanks SOOO much for any advice you can give us! -- Jodi from PA

Answer
Hi Jodi:  This is a bit of a mystery.   I will start with some random comments from questions I found in your letter.  

First... I always recommend that you supplement the diet of plecostomus and other bottom feeding fish with algae wafers.  There is hardly ever enough algae in a tank to sustain them for long.  

I had the sad misfortune of visiting one of the name brand pet stores a few weeks ago because I needed a uva uvb bulb for my turtle and we were out of them at my work.  I can not even begin to tell you how sad that place made me.  I am so fortunate to work in an aquarium store that takes the best care of its fish.  Many of the fish in that store were sick and the fish expert there was not an expert his advice to other customers was just horrible, wrong and very misleading.  I have made it a point to order things off line as opposed to having to go back to one of those places.

My advice to you is to find another aquarium store and not a name brand store and go have your water tested again.  I suspect that part of your tank problems are coming from water chemistry.  I am a little concerned about your feeding program.  How much are you feeding then and how often?  I feed my fish twice per day but no more then they can eat in about three minutes of time.  If there is food left over after about three minutes then I cut back on the amount until there is no food left over.  I also give them treats in the middle of the day or in the evening... brine shrimp and blood worms.  I would be curious to know about the pH value of your tank before and after a water change.  Changes in pH can be very deadly to fish... so a test kit that is very accurate is a good investment.  Also... I don't change my water weekly... I change it when 1) the gravel in the tank needs to be cleaned, 2) I have a water chemistry issue, and 3) about every 30 days a week after I have changed my filter media.  

Basically either the fish are not getting enough to eat or you have a water chemistry issue... find another place to get your water tested and let me know what the values are... dave