Why Are Some Of My Fish Skimming The Top Of The Water

[dawsss18]

I have a 400 litre tank (roughly) its been up and running for about 6 months in my house and was previously set up in someone elses house before me with the same filter.
 
Yesterday I decided to swap my gravel for sand. in the process i also did roughly a quater water change. 
 
Now my 4 severum, and two cichlids are skimming the top of the water, why are they doing this? are they ok? 
 
My pleco, fire eel, golden gourami and tinfoil barb are all acting their usual self. 
 
many thanks in advance

 

[Lunar Jetman]

Have you checked your water stats? Disturbing the substrate of your tank could have released all sorts of nasties into the water so you might need to carry out more water changes.

 

[dawsss18]

Have you checked your water stats? Disturbing the substrate of your tank could have released all sorts of nasties into the water so you might need to carry out more water changes.

 
I don't have a water testing kit, however I will take some water to my LFS and ask them to do a test tomorrow, as obviously they are closed now. 
 
I'll do another water change now, how much would you suggest? a large water change or small water change

 

[BerryAttack]

i would do a large water change and add a bubblier to the tank.. my fish only go to the top when there is too much CO2 in the water.. my tetras go first then the angels... could be having a lack of oxygen .

 

[dawsss18]

just to let you both know, i did a water change and i've just checked back on my fish and all of them are swimming like normal again now. Many thanks for your quick responses.

 

[Ninjouzata]

Glad they are back to normal but definitely suggest picking up a liquid test kit for yourself so you don't have to worry about if the lfs is open or not

 

[dawsss18]

Glad they are back to normal but definitely suggest picking up a liquid test kit for yourself so you don't have to worry about if the lfs is open or not

yeah i think i will defiantly get one, thanks