No matter how stable I keep my water conditions, and how much I clean my tanks, and have a high-er pH and listen to all other internet sites, I can't keep one guppy alive for more than two weeks.
ILOVEBETTAS:
I have been there, done that and got the tee shirt.
No telling the $'s I spent on high end show guppies only to have your experience.
All I can tell you is what is now working for me.
My main tank has pristine water with Gh typically less than 5, nitrates typically less than 15, Ph typically 7 and temperature typically 78F.
When I purchased the several batches of show guppies and placed them in my main tank I had a few survive for a year but most were your "two week casualties".
I set up two 5G Eclipse's.
The last "batch of show guppies" were of two "flavors" and were only fry when they showed up.
I placed one "flavor" in one tank and the other "flavor" in the other tank.
These aquariums have very, very rudimentary filtration but I do 90% daily WC's from my main tank (which not only provides "high quality water" but also acclimates the guppies for induction to my main tank).
I have diffused air which provides minimal agitation of the surface in the two 5G aquariums.
I also placed some floating plants (not "real dense" but only covering about 40% of the surface area) in the two 5G tanks.
Lo and Behold!
Virtually all the fry grew to adults very quickly.
The ones which I placed in the main tank are still thriving (I have floating plants in the main tank also).
The females in the two 5G's as well as the females in the main tank are bearing and the fry are not only surviving but also growing rapidly.
Additional Items
One
I have (or try to have) 1 male/3 females.
Two
I try to feed three times daily in small amounts.
My feed consists primarily of my home made recipes (beef heart, shrimp and fish with condiments including but not limited to veggies and garlic) as well as crushed up TetraMin Tropical flakes, frozen brine shrimp, frozen blood worms and live red wiggler worms.
Three
In one tank I have 50 baby (but growing rapidly) red crystal shrimp and in the other tank I have 6 baby (but growing rapidly) bristlenose Plecos for some "cleaning up".
TR