African Cichlid behavior

Question
About 8 weeks ago I started an African Cichlid aquarium.  The aquarium is a 75 gallon rectangle.  I started the aquarium with 4 African Cichlids, 2- Acei and 2-Electric Yellow Labidochromis (no clue on male/female mix- not smart enough to figure that out).  I know... you are not supposed to start a new aquarium with these fish.  But I started the aquarium with filter media from an established aquarium and used API Stresszyme to jumpstart the cycling process.  PH is a consistant 8.2, Ammonia and Nitrites are near ideal.  Nitrates however are elevated to near 60ppm, which I am addressing with aggressive water changes (I was purposely holding off on water changes, until the tank stabilized- this may have been a mistake however).  Now to the question-

During the entire time all four fish have done well.  But recently the Electric Yellows have changed behaviorally, and I trying to understand why.  Firstly, one of the Yellows is smaller than the other- but both are exhibiting changes.  The larger Yellow has started to dig and maintain a ~3" hole in the gravel under some rocks in the tank.  The smaller Yellow has started hiding almost constantly- in an inverted position (either up or down) behind the aquarium heaters of all places! (note: I have two heaters, one in each corner)  The larger Yellow has also recently become much more aggressive towards the smaller, I've noticed.  

These behaviors all started about a week ago- but continue. I'm trying to discern if there is some form of breeding behaviors at work here (seems unlikely in an 8 week old aquarium) or if something completely unknown to me is at play.

Thoughts?

Answer
Hi,
It is quite difficult for me to say what is happening.  It could be that the males are fighting over a female which is why there should always be more females than males.  When one is stronger, it can kill another, which could be the reason why the other is hiding.  Lack of rocks, overturned pots could also be a reason.  With African cichlids, you must change your decor often so that they do not get too comfortable with their territory.  You must also do this when adding new fish.  Your nitrates are high, and you should have no ammonia, or nitrites in your tank.  The best way to cycle a tank, is not using fish, but buying a used sponge from your pet store, squeezing out the access water, and putting it directly into your filter, letting your water run through it until your chemistry is right.  You only add the water that has evaporated.  After, when your water has the right chemistry, and you need to clean your filter, you always rinse your sponge in water you have removed from the tank.  A sponge holds good bacteria that fish need to survive.  A sponge is good until it is old, ragged, and falling apart.  Always make sure there is lots of water movement in your tank, so that they don't lack oxygen.   Add fish slowly.  Never add too many fish at once, as this would cause your chemistry to change killing off all your fish.
Observation is always the key with Cichlids.
I hope this will help.
Lynda