Shrimps dying, any cause of worry?

[dkk08]

Hi guys, just came back and again I found some dead shrimps, is there a cause to worry as this is the 3rd or 4th time I came back from work and I found a few dead shrimps, no. of dead shrimps found each time is 2-4 of diff sp... yesterday added 20 Malayan shrimps(lotsa of them r preg somemore!!) and 11 tiger shrimps... the following is some specs of my tank...

2ft Tank: cycled for abt 7-9 days before adding livestock
CO2: DIY C02 24hrs (bubble rate dunno coz DIY, unless someone can teach me how to count or can I use a diffuser or bubble counter?)
Light: 1x55w T4 PL, on for 10-11hrs
Plants: Java Moss, Taiwan moss, monosolenium tenerum, Red Tiger Lotus, C.wendtii brown, cirinum calamistratum, glosso, hairgrass, viallis, duckweeds and tiny water lettuce
Fert: Super Roots Monster and LushGro aquatic (once during cycling)
Live stock: 8 Pygmy Cories 1-1.5cm (C.habrosus), 3 Panda Cory 1.5cm, 4 Clown Loach 1-1.5cm, 3 1.5cm Oto, 20-30 Cherry shrimps, 20 Malayan shrimps, 20+ Tiger shrimps, 10 Mosquito shrimps (Shrimps population as when I bought them)
O2: Only at nite
Temp: 21-25.5 deg cel with DIY fan on 24hrs
Death rate: 2-4 each day
Current death report: 4 Mosquito Shrimps, 6 Tiger shrimps, 3 Malayan shrimps

Is my tank overstock? I've been wanting to ask but sometimes I forgot to take out my O2 in the daytime when I go to work, so its in there 2gether with the DIY CO2, does that mean the CO2 have been dissipitate (meaning no effect?) Pls advice bros

[dkk08]

ok I forgotten I actually has a set of Aquarium Pharmaceutical Freshwater Master Test Kit, just ran some tests n here's the results:

PH: 6.4-6.6 (I'm a little color blind!)
Ammonia: 0.25ppm
NO2: 0
GH: 100-200ppm (again I'm a little color blind!)

wat say u guys?