I am not talking about a "small" filter...I am talking about one that is designed for the size tank you have. For example I have a 75 gallon full blown planted tank....I have an Ehiem Classic 2215 its designed to filter up to 93 gallons of water. All my plants and fish are thriving....I don't need a bigger filter its just not necessary.Why would I spend my hobby dollars on a bigger filter when I could spend it elsewhere ?
Up to 93 gallons is the key phrase. Eheim doesn't disclose the same information on the american models as they do the european models, but if you look up the classic 350, which is the euro version of the 2215, you'll see the following.
The 350 is rated "for aquariums of about" 120 liters (31 gals) and "for aquariums up to approx." 350 liters (92 gals).
The media capacity is 4L (~1 gallon) and the pump output is 620 liters (164 gals).
Link here: https://www.eheim.com/en_GB/products...rs/classic-350
Interestingly the ratings for the pro filters are much more conservative. The 2073, which is rated for up to 90 gals, has a flow output of 275 gals. That's nearly double that of the 2215. Media capacity is only slightly larger than the 2215 at 1.3 gals. Yet those filters are rated for the same size tank by the manufacturer. This is why one can't just look at the box and pick a filter. You need to decide how much turn over is enough. 5x is generally a good aim, but sometimes you may want more.
Now it gets a little more complicated if you want to really drill deeper. The pump output numbers don't account for any fluid losses as a result of the media, especially dirty media. Those numbers are strictly how much water the pump puts out. So that 5x is maybe more like 4x or 3.5x the aquarium capacity after you add media. Not a huge deal if you start with a filter that has 5x the capacity of your tank as you would if you're running the 2215 on a 31 gallon aquarium. However, in your case you're running that filter on a 75 gallon so you're starting at only 2x before you even add media, less if you're running that filter on a 92 gallon tank. After media you're at maybe 1.5x or even less.
It's certainly doable as you've demonstrated, but I wouldn't call it ideal by any means. A lot also depends on the tank as was already pointed out. It's one thing if you're keeping tetras and cory cats and a very different story if you're trying to keep 4 discus. The plants help too of course. But once you get to such a low turnover rate by the filter you are pretty much forced to do large water changes every week. You can't let the filter go 3 months without cleaning it. If you are diligent about your maintenance you can certainly pull it off, but I'm not. Some weeks I just don't have the time and for me the few extra bucks for the convenience of being able to miss a week here and there is well worth it. I think this is true for most people and that is why I recommend a slightly larger filter whenever possible. Overall the larger filter expands your options quite a bit. You can plant the tank more heavily before you run into dead spots, you can stock the tank more heavily, etc.