Livebearers: What am I doing wrong with my guppies?
 

I have had fish for 20 years and livebearers are the absolute hardest fish for me to keep and I don't know why. I have tried relentlessly over the years because they are some of my favorite type fish, especially the guppy but never have much success for very long. I have a 55 gallon community tank with 3 black neons, 10 neon tetras, 1 red tailed shark, 2 plecos, 1 cory cat and 7 or 8 guppies (at the moment). My guppies are never quite right and I'm always having to buy new ones. I buy from breeders so they are very healthy fish when I get them. Over and over they eventually become sluggish, clamped fins and develop fin rot. I have experienced the same problems if I have tried mollies or platies as well. I continue to have lots of success with any other type of fish. I haven't lost one neon tetra since I bought them months ago and in the past those had always been difficult fish for me to keep. Some of my fish in this tank I've had for years and they've always been so healthy. Why do I have such poor luck with guppies? I'm determined to make this work but I've spent so much money over the years only to keep loosing them.
Here are my tank stats:
Nitrate: 20
Nitrite: 0
Hardness: Soft
Alkalinity: 120
PH: 7.2
Ammonia: 0
We have well water and I have had this tank set up for years.

try doing a 20/30 percent water change every week and make sure the current in your tank is not to strong also with mollies try adding some salt to the water. i have 7 teaspoons of tonic salt in my 30g community tank with tetras and loads of other fish including livebearers, after i added the salt and started doing more frequent water changes my livebearers started to get a lot healthier and producing more fry.

salt
 

I always add salt with the wather changes and I've been adding 1 Tablespoon to every 10 gallons. I try to be cautious with the salt since I have the tetras but perhaps I'm not adding enough. Is I Tablespoon to every 5 gallons per water changed enough and not harm the tetras?

how about add no salt since you have tetras, livebearers dont need it and you shouldnt need to add salt unless its a livebearers only species tank or a brackish/saltwater tank.

Maybe your water´s probably too soft ? Mollies and Platies really like it hard and I guess the Guppy as well.

As someone mentioned, frequent water changes increase the healthiness of your fish. All the symptoms you´ve mentioned are a sign that something (waterparameters) in your tank is wrong.

How´s the red tail shark´s behaving ? Does he bother the Guppies ?

i've NEVER had trouble with livebearing fish and it may be down to water which is hard and alkaline i've had limited success with softwater species if your water is natural soft try coral sand as a substrate

dont do coral sand as a substrate, use some crushed coral in your filter to help harden your water.

Set up a separate tank for your gups. That way, they can have the hard water they like and your other fishies can keep the soft water they prefer.