water clarity

Question
hello,
I have kept aquariums for years and water quality has never been an issue, I will admit in the past I didn't concern myself with testing water because I never had any health issues with fish and water was clear.  I set up a 150gal freshwater aquarium 8 months ago that i had(I had it set up 3 years ago with three hang on back filters with no problem#.  I have the tank now on a fluval FX5 filter with 3 lbs of carbon and about 3 lbs of bio stone, no buffer pads in it-it slowed down to much. I also have a TMC-V2 1000 bio fluidized sand bed filter rated for about double of my aquarium capacity that has been running for a month according to manufacturers specs. I tried running two other small fluvals with the large one before the FSB filter but my water to this day is tinted yellow. I had the timer for my lights on for about 14 hrs a day and have learned that is too long and have left them off for about 4 days now. I don't have an algae problem in the tank and could go 2 weeks before i clean the glass at all.  I don't have any plant life and the fish load is rather small with about a dozen fish with the largest being an arowana 8 inches long, other than that the other fish are 5 inches and well under that. I do have a 3D background made professionally available online. I also had my tank top glass on all the way across but removed the mid section to help lower the temp and help gas exchange. I also raised the outlet for my fx5 to create even more water movement and aeration about three weeks ago.
I did quick 5-in-1 test and nitrates and nitrites were really high on the color scale, with a rather low PH and #color scale put it at a low 6 high 5). The PH really threw me because my last set up was a successful African cichlid set up using same water source. I will also say it has affected fish differently, I lost a bunch of young two inch frontosas after they triple their size and some other cichlids but buffalo head pair has thrived and spawned a few times already with the young being kept in one of those fry net boxes hung in the aquarium. The young are dying off one by one it seems as well after a good few months.  I am willing to do what it takes to make this tank successful and appreciate any advice you may have. If there is any other information that you may need to help me I will do what I can.

Answer
Hi Brett,
Even with the greatest of filtration... No filter can truly remove all the pollution in your tank water. Which is why your nitrates are really high. I'm concerned why nitrites are present as well. As you know ammonia or nitrites should really never be present in your system. There may be a buildup of too much waste for the beneficial bacteria to handle.

A low pH is also a red flag that there is a high level of dissolved organic compounds in your aquarium. Decaying matter typically produce acids that over time, lower the pH.

Unless I missed it somewhere... I don't remember you mentioning how often and at what percentage you changed the water in your tank.  

The high nitrates and nitrites and low pH -- (plus buildups of other pollutants we can't measure for) are -very- likely the reason the fry are dying off one by one. They can only tolerate/adapt to it for so long. Baby fish are most at risk.

If this tank has been without a water change for a month or more I would recommend starting out slow and gradual. Do a 20-25% water change one day...then another 20% water change the next day and so on until you have gradually exchanged the entire volume of water over in the tank. We dont want to do a sudden big water change if the pH of the tap water is higher than that of the tank to prevent the fish from being shocked by the sudden change.

Since this is likely the case here be sure to keep up with regular partial water changes and make sure to rinse mechanical filter media regularly as well.

I hope this helps solve your current tank issues!
Good luck!
Susan~