Yes and no. Depends.
A fish-only may benefit from a filter in the system, but!
Here's a list of useful facts to consider.
1. ONLY live rock and sand can break somewhat-harmful nitrate all the way down to harmless nitrogen gas. Bioballs can't and a filter can't. They take waste down as far as nitrate and stall there. Nitrate builds up and up until there's a cleaning-out of same.
2. Fish can tolerate higher nitrate than corals can. They'd probably rather not tolerate it,...

Run it. Now. To see where you are.
If ever things are going squirrel-ey---test your alk.

...Then test it weekly, during your first half year. Bi-weekly thereafter. This is for fish-onlies AND for reefs both softie and stony. For all tanks. Alkalinity is THE most important test for a reef, once you pass your cycling hurdle. And for a fish-only, alk controls so very much regarding fish health, just consider it an absolute essential. If it's right, you've got a major part of fish-care...

Freshwater hobby to Saltwater hobby: the differences.
1. salt, obviously. The salinity level of a tank should not bounce around. Use of an ATO (autotopoff) is recommended.

2. ph becomes a fairly useless reading: track alkalinity instead. PH rises and falls during any single day. Alk is tightly locked to the calcium and magnesium in your tank. If one depletes, readings will start to fall. Keep the mg around 1200-1300, keep the calcium about 420, and the alk around 8.3-9.3 and...

Coralline: what good is it, what harm is it, and do I want it? Informational.
Coralline is the purple-violet stuff on your rocks.
The bennies of having it on all your rock: a good coralline coating principally means other, more objectionable algae can't grow there efficiently. It probably (my guess) does limit efficiency of water reaching inside your rock, but if you have holey rock, you have plenty of exposure, including the part sitting on the sand, where this stuff won't grow....

Do I need GFO? What can it do? Informational.
GFO is granulated ferric oxide. Iron, in other words, in a particular form.

It can be added in a bag for a very small tank, BUT---to really be effective, you need a GFO reactor. These, fortunately, are comparatively cheap, and require only a small pump to run. They also come in more than one size.

Here's what they do:

A GFO reactor sucks up phosphate, which you do not want in your tank---because it...