Help! Issues in tank and with fish

[Fishmandamo]

Morning everyone,

I hope you can help me.

I woke this morning and have to gone to feed my tropical fish and discovered all the fish close to the top of the tank. I instictively knew this was likely a lack of oxygen and did a one third water change, to which the fih seem to have settled. However i have studied the tank to look for problems as to why this may have happened (i only did a fortnightly water change on wed - 20%) and discovered a few things:

• Some of the fish seem obssesed with nibbling the gravel in one corner specifically, made me wonder if perhaps a fish has died and they are eating the body - no signs of a body though, perhaps this caused contamination and thus the fish at the top of the tank?
• I noticed one tetra seems to look ill. few white spots, one large and his body is a little enemic
• Another tetra seems to have almost a blood stained belly

I attach pics of the fish in question

Other things to note:

• Tank is well established
• tank was seemingly healthy until this am
• Fortnightly 20% water changes, the last of which was wed.
• Additional gravel was added a week ago - not lots, just a few handfulls as the tank had become a little bare in areas (pump/cleaning)

Thanks all

 

[Kolykaf]

Did you test the water? Perhaps a fish did die and caused an ammonia spike. Another possibility is that you experienced a bacterial bloom which caused oxygen levels to plummet. Had you made any other changes like removing some gravel/decor or cleaning the filter? Either of these can cause stress which allows fish to become susceptible to disease/infection. It’s kind of difficult to say what’s going on with the neon. Maybe a fungal infection or columnaris. I’d isolate him. Meanwhile, test your water, keep it clean and watch your other fish for similar signs of infection. You haven’t recently introduced new fish, have you?