Mysterious Death of Shrimps?

Hi, I have been keeping shrimps for around 8 months nows. With cherry shrimps initially in a small tank and moved on to a 2 ft tank. The reason to move was because my small tank bedding was contaminated with nitrate and alot of shrimps died. and mainly, i wanted a bigger tank to enjoy the shrimps.

after changing, i kept cherry shrimps, sakura shrimps and tiger shrimps. bought them all at crs haven at tampines. however, for the past 5 months, since i changed to my 2 ft tank, shrimps start to die every week. at least 1 shrimp will die every week, sometimes 1 each day. for the past 4 days, 1 died each day.

was wondering if anyone knew this is normal or if there is some problem with the tank? i asked crs, they say it's normal for shrimps to die now and then... ??

here's my tanks env.

2 ft tank, normal filter, 1 fan.
temp - 28 degree
nitrate - 0 to 5.
water change - every 2 weeks once, 10% change
water topup every week due to evaporation
feeding - every day 1 pellet.
total number of shrimps - 20. 7 cherry, 5 sakura, 8 tiger

any help would be greatly appreciated. thanks.