[Roc]

This is also posted in water paramaters

Ok 1st let me state what I do know

20 gal 24X12X16
2 X 55 watt 10000K on for 6 hours a day
Topfin 150watt heater set for 79 degrees
Prussuized CO2 about 2 bps
HOB Emperor filter

Ferts:
KNO3 -1/4 teaspoon every other day
K2SO4 - 1/4 teaspoon every other day
KH2PO4 - Mixed 1 1/2 teaspoons with 240 ml of water add 4 ml every other day
Plantex CSM+B - mixed 1 tablespoon with 1 cup water, add 4 ml every other day (not same day as other ferts)
Tank up and cycled since June 06
heavly planted:
10 X Rotala sp. 'Colorata'
10 X Lagarosiphon madagascariensis
30 X Myriophyllum matogrossense both ‘Red’ and ‘Green’
2 X Cryptocoryne wendtii 'Brown'
6 X Hygrophila polysperma 'Rosanervig'
10 X Hygrophila polysperma
10 X Microsorium pteropus ‘Narrow leaf ‘
8 X Nesaea crassicaulis
1 X Ludwigia inclinata var. verticillata 'Cuba'
1 X Nymphaea lotus 'Zenkeri'
5 X Blyxa Japonica
6 X Alternanthera reineckii
Blyxa japonica
Vesicularia dubyana
Vesicularia sp. ''Christmas''
Riccia fluitans

Water Parameters

Tap water:
KH - 40ppm
PH - 7.5

Tank Water:
KH - 30 ppm
PH - 6
Amonia - 0
Nitrate - 20ppm
Nitrite - 0

Drop checker shows a perfect balance (although the test do NOT)

Ok here is the issue, I have 3 Clown Plecos in this tank all been in it for about 6 months all are fine, I have 15-25 Red Cherry Shrimp that have been in tank for about 2 months all fine, I recently tried to add some Neons 1st 12 of which 8 died (I thought I had just gotten some bad fish as they died about 2-3 a day) so I talked to the store and they gave me 10 more (they died 1-2 a day). At this point there is 1 neon left, none of these fish showed any issues before there deaths.

Any ideas?

Obviously I don't want to add more fish until this is figured out, I didn't dose today just in case someone thinks that could be it.