If you have an Oscar chances are it digs down to the plastic of your UGF.. They are for the most part considered obsolete, especially with cichlid keepers.. U see, when any of your fish dig to the plastic or pits close to the plastic floor of your UGF that is where the water chooses to escape first.. As water always seeks the easiest way out

Hence leaving the rest of your UGF coverage Useless

And stagnant.. Anymore I just say they look nice LOL
Personally I like the Emp 400 though if I were going to stay with a 55 for the O.. I would go with 2.One will work for just those two fish but 2 is better

. You are going to get allot of good suggestions with a few choice name brands.. One will be AC or Aqua Clear.. Which I have no experience with but hear allot of good things, And then you will hear about Canisters

Most likely Fluvals ...
Oh also I would do a water change every week.. Schedule a day and that is your change day... This is disputable based on allot of things.. But change a minimum of 30% and Vacuum every time

Over more time you will find what works best for your tank parameters.. And then it will just be natural.. \
Just my two cents to hopefully push you in the right direction.

Oh and you are also going to get flamed.. So take what you need(content) and leave the bull..
