strange behaviour

I have a newly aquired mixed tank of africans.
I have a 77 gallon tank with 18 mixed africans and 5 goldfish (used to cycle it with (the 5 left that survived there was 15) because since has been cleaned).
The tank was previously a pirhana tank for over 3 years so biologically its good i think.
I tested the ammonia=0
ph=7.2-7.4 and i just added stuff to increase the ph.

they are all (africans and goldfish) swimming at the top with their mouths skimming the surface. im not sure if they are attracted to the light or if this is a strange behaviour related to water chemisty. i do not have a nitrite or nitrate tester kit.

i am not new to fish keeping. been at this for years with healthy tanks. I am however new to african cichlids. I have been researching them for the last week and am aware not to mix the lakes but my lfs had them all mixed so i bought my faves i expect casualties. they are all around 2" i have no clue what species or if m/f.
i plan on feeding them feeders when their big enuf. i like a violent aggressive tank. other than that i take care of them very well and do my weekly water changes. what are your thoughts and advice any and all is appriciated. i am a member of a piranha site just like this one. great site i have learned alot. thank you and thanks for your comments.
should i be concerned about there behaviour about swimming at the top??? *** only had them for a day. and checked ph and ammonia 3 times.