fast gilling and turning dark help needed ASAP

[prats]

i have tank
dimensions 48"*18"*24"ht
bare bottom
30 C temp
WC 60% daily
PH 8
ammonia 0 ppm
sometimes some of my fish breath rapid so that they don't close the gills completely between two breath, they turn dark too and clamp the fins too at these times they take the feeding thrice
1 tetrabits
2 hikari bio gold
3 earthworms live
as they are 1.5" juveniles
their feces are dark brown and sometimes reddish brown
i have sponge filter, ehiem 2213, and two heaters one of 200watts and other of 300watts
all the above prob occurs sometimes and goes of but it frequeint

[captainandy]

since the problem is repeating itself suggests gas exchange problem. Increase circulation and add fine mist bubblestone
Check usual chemistries: nitrite, pH, nitrate

Also, does the rapid breathing occur after water change?

[jimg]

definitely check the nitrites

[prats]

since the problem is repeating itself suggests gas exchange problem. Increase circulation and add fine mist bubblestone
Check usual chemistries: nitrite, pH, nitrate

Also, does the rapid breathing occur after water change?

yes after water change it does happen too and turn dark