My fish are swimming sideways

I have two good sized angel fish which I have had for about over a year now. I notice that sometimes they swim sideways, it appears maybe their equilibrium is off. They dont always swim that way, just every so often during the day i notice them floating lopsided or swimming sideways.

Can anyone advise at to what may be the cause of this? And if it is a problem, how can I cure it? thanks.

Sure...

Tank size is 30 gallons...I only had the 2 angel fish by themselves for almost a year now...there were 2 other ones but they died a while back...I just added 2 tetras and a pleco on monday, so basically the 2 angel fish were living by themselves in the tank for almost a eyar...

They have been occasionally swimming sideways for some time now...

I maintain the tank well...I recently changed the carbon, and the sponge filter in my Fluval 304 filter...I change the water every so often, the temperature is around 78 degrees....PH balance is good, no ammonia or nitrates....have an air stone in the tank...

I cant say for sure, but at random times i look in the tank and I will see the angel fish swimming lopsided or sideways...Then other times they will be swimming straight.