Need more info
How long was your tank running (total), with the one aq50, with full stock, and with the two aq50s?
Did you simply add a second or replace one and add one?
What procedure did you use when adding second filter? (How did you transfer original biological filtration...was it left out, put in tank water to seed, left as it was)
Did you already have the ammochips or is that new? Where did you place them and if in one filter, was it the original or new?
Here is why I ask: You have had high ammonia levels all along, which would be explained by a tank that was cycling, never fully cycled, new additions (fishies), OR and this is another good question: what test are you using? Many tests like API's, test for ammonium as well and could be showing your level of ammonia as high when it is really ammonium.
I think that you may have inhibited the development of your biological filtration in some way. Ex: if you have added a filter and are removing ammonia before it reaches the filter, this will slow the growth of the baceria you need on that filter. I, personally, have found that artificially removing ammonia is usually a short term solution for creating a long term nightmare. A tank that is not overstocked and fully cycled should not require the addition of ammochips. If you need to grow your biological, you will probably need to remove the ammochips. If your levels of free ammonia (test with one that tests for ammonia only) begin to rise too much, try small water changes only...but you need to allow your biological exposure to the full load of ammonia that is put out, or you will not stabilize. Then you will have to allow the nitrites to build enough for the same reason. You may be in for another cycle, but that is the only way to establish a healthy and resilient tank. Also make sure you never remove your biological filtration without properly seeding a replacement (though your redundant system will help with that). You might find someone with an established tank and see if you can find an additional source of biological material to assist in the process. If you can find a temporary home for some of your fish during the process and then slowly add them back, it would likely save lives. Doing frequent water changes on a tank that has not established a full bio can also keep it in 'mid-cycle' and keep you always busy 'maintaining' for your tank, but your maintenance will always be subpar to a fully established system. Sometimes you just have to let it run its course and it will stabilize. If you have losses, it may help to consider that the fish and system that is left will be the 'fittest' and will prevent larger losses of fish and money and time in the future.
