die off

I started with 9 platies; I'm now down to three. I've had at least one platy die each day, a couple days I lost two in just one day. Heated tank (75F), sponge filter, 20 gallons, lightly planted, over 2 years old. Parameters are as just tested today: Alkalinity 40 mg, ph 7.6, nitrate 0, nitrite...

So, I've got a 29g tank, already running for 4 months. Couple of months ago I bought a dozen neon tetras and within 2 days 5 died. Columnaris, NTD, or too much stress, never found out for sure. Fast forward to yesterday. I've had a planted and cycled "quarantine" tank running for 3 weeks now...

Please help! I have two 54 gallon freshwater tanks that I am really struggling with. Have had the tanks for a month, added new fish to both tanks a week ago. Tank one: 7 guppies, 2 silver dollars, 1 betta, 1 pleco PH 7.0 Ammonia 0 Nitrite 0 Nitrate 0 Tank two: 3 goldfish originally, added 5...