Help- Guppies always getting Tail Rot

[Speedfox]

Hi,

I have a 130 liter planted tank with around 20 all male guppies. They all get along nicely and there is no bullying. With API test kit I get 0 Ammonia, 0 Nitrite and 5-10 Nitrate. PH is 7.8. Temperature is 79F/26C. Water change 20% once a week using Seachem Safe to remove chlorine. I wash the filter material in old tank water when it gets too dirty. The lights are on in the tank for around 8 hours per day.

In the morning I give them half a cube from a frozen blister pack (tropical quartet - shrimp/daphnia/cyclops/bloodworm/tubifex)
In the evening I give them some homemade vegetable only food similar to: https://www.youtube.com/watch?v=N_uksybDyxM

Does anyone have any ideas as to what I am doing wrong? They seem to slowly 1 by 1 get Tail Rot and die like 1 per month or so

[TropicalFishForums]