The % biomass of the plants needs to be matched to the bioload, not the size of the tank.
But it's okay to do a bit of overkill.
Same deal with any filter system, the more, the better I figure.
Rare cases perhaps otherwise, but those are rare.
I'd fill the 4ft tank/sump, with 36" full of the plant "pots", lava, zeolite etc, then a pair of large sponge filter blocks and gate right before the intake for the return pump.
Since this is a fish only system, you can use a small valve and a venturi suction(or small air pump) for aerating the return the water. If you do not like the mist, you can use an air line valve to adjust and stop it for viewing etc, but when not, leave it going to keep the O2 high. Adjusting it down to lower aeration also reduces any noise, but I think the return pump will be louder and it'll be under the tank etc.
If you have high plant biomass high fish loading, high bacteria, you need high O2 also. Not just for the fish breathing, rather, to help the bacteria and plants process all the waste and cycle it fast. This helps the fish and adds O2 for them(aerating the return line).
Make sure to install and hard plumbed drain and refill valve for this tank.
I doubt you live in a small apartment, so do this now at the planning stage.
Add the water fill(hot/cold) and drain lines(this can go out into the yard or landscaping for irrigation/ground water recharge etc) directly to the tank.
All you do is turn a valve to drain any % of the tank you want, then turn another to refill it.
This makes water changes very easy and you need little motivation to do a water change from then on. You will save your self hours of labor and be more likely to care for the tank. Also, your other hobby friends will envy the set up.
Even if you hire a plumber to add the lines, it's still pays for itself in 2 months.
If you own the home, you are insane not to add such a system to larger tanks.
If you are concerned about the Electric bill, a skylight or placing the sump next to the window and using less light or only during winter etc kight work just fine also since the filter system is in the other room.
This way you get free light and use the wastewater for irrigation.
You can also use the plant trimming for compost or sell them etc.
I would try and make the filter look like a nice Hydroponic style garden.
Those clay balls they sell for media work well also.
You can take 3" PVC pipe and cut into 18-20" long pieces and glue those test 3" caps on the bottom drill a few small holes for drainage and fill with the zeolite, clay balls, lava etc and plant the peace lilies in there. Some small air line tubing and a small pump will slowly trickly the water through each 3" dia pipe.
This is pretty easy abd a very high % success rate method.
If you are competent enough to set up a large tank, do sumps etc, then this is cake work. As long as you have plenty of plants, light, and water, they should do pretty well.
Regards,
Tom Barr
Regards,
Tom Barr