If you want to boost that Rena a bit at some point you can either find a top from a xp3 or a bottom from a xp3 and create a hybrid filter. With the xp3 top and xp1 bottom you'll have a ton of pull thru the filter, and with the xp1 top and xp3 bottom you'll get a big space for media. Either way you win if you do it at some point.
If it were me I'd put a Rena xp3 on the tank and a powerhead or Koralia on it with the xp1 for backup.
I've got a huge load on my 75, and I use two AC110's. They are more than enough by themselves. I wouldn't even think about adding a third filter along with them.
Thanks for all the responses everyone and yes I thought about the thickness of the background and filters. That is why i'm going to go with one of the thin 1"-2" versions that will not hinder the ability to use HOB filters. The XP1 I already have and that is why I threw it into the equation, however I could put it on my 29 gallon tank instead and take the Fluval C4 off of it and put it on the 75 with a AC110 and Aquatop.....got me thinking again lol. I really like the ideal of the UV and since I got one on my 220 with the Odyssea CFS-700, i'm hooked and they work.
I always like to have more than one filter on my tanks encase one stops working you still have something there running. An fx5 and something else is really overkill for this size tank in my opinion even though I do know you can use it.
two ac110 best bang for your buck, but if u really like the aquatop go for it i dont like it though cause it is a bother to open and clean and restart again
