Fish Dying En Masse - Help!

[Joey99]

Tank size: 200litres
pH: 7.5
ammonia: 0
nitrite: 0
nitrate: 5
kH: ?
gH: ?
tank temp: 23 C

Fish Symptoms ( include full description including lesion, color, location, fish behavior):
I've been away for a long weekend , leaving neighbours to feed the fish for me. I've just returned to find 6 barbs and 1 danio dead and the remaining barbs and red table shark seriously ill. The tank was full off green algae after only 1 wk. The water spray was broken - water pumping out the end rather than in jets across the tanK. I've done
All water tests and all seem fine son Wondering if it's because of the water pump - could it be a lack of oxygen?

Red tailed shark is gasping on the surface of the water, and largely upside down - he seems to have no strength to swim. 3 albino cories are sitting v still on e ottom of the tank - no foraging and with some occasional darts up to surface for air. Bristle nose plec is on side of tank, but is making sporadic visits to the surface. 3 small gouramis are swimming around and seem to be unaffected.

Volume and Frequency of water changes: have just done a 30% water change, previous water changes on a c. Fortnightly basis

Chemical Additives or Media in your tank:

Tank inhabitants: 4 danios, 10 barbs (tiger, green tiger, albino), 3 bronze cories, 1 red tail shark, 1 bristle nose plec, 3 gouramis

 

[ShinySideUp]

Tank size: 200litres
pH: 7.5
ammonia: 0
nitrite: 0
nitrate: 5
kH: ?
gH: ?
tank temp: 23 C

Fish Symptoms ( include full description including lesion, color, location, fish behavior):
I've been away for a long weekend , leaving neighbours to feed the fish for me. I've just returned to find 6 barbs and 1 danio dead and the remaining barbs and red table shark seriously ill. The tank was full off green algae after only 1 wk. The water spray was broken - water pumping out the end rather than in jets across the tanK. I've done
All water tests and all seem fine son Wondering if it's because of the water pump - could it be a lack of oxygen?

Red tailed shark is gasping on the surface of the water, and largely upside down - he seems to have no strength to swim. 3 albino cories are sitting v still on e ottom of the tank - no foraging and with some occasional darts up to surface for air. Bristle nose plec is on side of tank, but is making sporadic visits to the surface. 3 small gouramis are swimming around and seem to be unaffected.

Volume and Frequency of water changes: have just done a 30% water change, previous water changes on a c. Fortnightly basis

Chemical Additives or Media in your tank:

Tank inhabitants: 4 danios, 10 barbs (tiger, green tiger, albino), 3 bronze cories, 1 red tail shark, 1 bristle nose plec, 3 gouramis

Doesn't seem over-stocked so lack of oxygen is unlikely IMHO unless over-feeding has taken place. I'd change more water than 30%, perhaps another 50% or more. My suspicion would be acute over-feeding (neighbours are usually hopeless at fish-keeping) so vacuum the bottom while you do the water change but don't wait two weeks before you change more water; personally I'd do another water change the following day.