Problems In Tank, Help Needed

[bramswell]

I have had a 20lt shrimp tank set up for about 6 months (aquael) with quarts sand, small piece of wood, moss ball and some small plants and java moss.

temp 24'c
amonia 0
nitrites 0
nitrates 5
20% weekly water changes using JBL biotopol c (removes copper)
no fertalizers used
hikari shrimp cuisine food

all has been fine untill about 2 weeks ago, i had about 20 cherry shrimp in there and added another 6 and then they started to die 2 or 3 a day until wednesday i then did a 50% water change thined out the plants and had a tidy up only had 7 shrimp left and within 3 hours had 3 more dead.
retested the water and all is fine all seemed settled with the remaining shrimp doing ok and still feeding.

last night i put a few of my tank bred guppies in with the shrimp and this morning they were dead and 1 swimming at the surface (transfered back to main tank and has picked up, seems ok now

the shrimp are still ok

the only thing i noticed was in the sand, there was some patches of black, cant remember what that is but a sulpher of some sort, but that has been there from about the 1st month of setting up and has been disturbed before and not had any issues but with the last water change i used my gravel vac and cleaned the through the sand.

i'm at a loss as to why this is happening, any suggestions?

 

[KittyKat]

Any pH or hardness fluctuations in the tap water? It does sound like a water quality problem, but as you say, not obvious what is causing it.

 

[bramswell]

no issues with ph
all my other tanks are doing well with no issues and they have water changes on the same days
the guppies i put back in the main tank have picked up again, so i would agree that it may still be a water quality problem i just dont know what

someone suggested gas from anearobic sand, can this be that harmful to livestock?

if thats the case would it be better to remove the sand and use small gravel

 

[KittyKat]

someone suggested gas from anearobic sand, can this be that harmful to livestock?

Apparently it isn't.. although I have to admit, bad cases of it do make me feel slightly queasy and I expect they must influence other life forms at least to some degree!

Have you tried stirring the sand more often? Also same can happen with gravel too so that's not a guaranteed solution.

 

[Joshwainwright]

Harmful gas does form under your sand substrate however it is not harmful unless it actually gets into your water. All that happens is a bubble forms, which then rises to the surface and pops. The only gas exchanged will be due to the movement the bubble created on the surface.