[Asteroid]

Is that 1500 LPH out of the box? Either way one filter should be more than enough to generate enough flow for co2 and dissolved ferts. You really don't need two filters as most of the filtration will eventually take place in the tank between the plants and surfaces mostly the substrate.

It sounds like you have nice aesthetics, why do you want to put more equipment in it. Just run the intake back left or right and the return on the same side but toward the front, so you get flow growing across the front and around the back.

 

[Asteroid]

1 filter would probably be enough but I am a big fan of redundancy, plus it also depends on the scape right? An open scape like @woody123 has above could get flow with 1 filter, but with a very densely planted "islands" design with 2 incredibly dense back corners or very hardscape heavy scape I would be stressing if the backflow will manage to push the flow back through the back. Having the additional filter then would be great.

Filtration is not really a big concern for me to be honest, like you said, the substrate and plants will do good job taking the no3 out of the tank. Beyond that, I always just fill up 2 out of 3 media buckets in the filter with matrix leaving 1 for mechanical filtration. This also also one of the reasons why I want 2 filters, 1,500 lph out of the box is great, but it is still a manufacturing statement. It could be lower, or higher, in practice. 1,500 lph is also based on the default filter media, so as soon as I fill up it properly with media the flow will get hampered a bit right? Then we still have the oversized reactor, which, if I recall correctly can slow down flow up to 10%

Yes, your right out of the box 1500 lph is not real world, probably 1/2 of that. The idea of 10x turnover though is to me fiction. There is no magic number for turnover. This number was out there when people only kept FO (fish only) tanks with big waste makers. This isn't usually the case with planted. The fish are smaller and you have the plants utilizing the waste. If one believes once the tank matures that most of the filtration is in the tank then what is so important about turnover? Aren't we really talking about flow? Lots of people say that about the type of scape, if it's heavily planted you need more. How would plant leaves stop dissolved ferts and co2?

I can actually make a case that it's better for waste to accumulate (other than if it's aesthetically off-putting) in the tank, because your probably removing it once a week with water changes. When it goes into the filter it's still part of the system.

Not saying there's a problem with large turnover, just isn't necessary. I've run many 4-foot tanks with 1x-2x turnover that were completely pristine and they were different types of scapes some heavily plant some mostly hardscape.

 

[Asteroid]

Wow, I expected it to be lower, but only half of is given frim the manufacturer? I agree with you on 10 times overturn not being a magic number. Though I do think the advice is mostly aimed at novices and intermediates that maybe don't have a full grasp the skills needed. I didn't know 10x was for FO tanks, I thought the logic actually came about for planted tanks. I've seen the effects of waves crashing into mangroves, the flow of water becomes drastically hampered or even becomes still water. I think the idea of man made mangroves was floated around to prevent tsunamis from getting to dry land. I always figured that's why planted tanks had higher flow, to make sure it pushes through even the most densely planted/built areas of a tank. Giving these "arguments" purely to learn and exchange info not to start a discussion.

Accumulating waste being a pro due to the added nitrogen? I will definitely do 50% water change every week with extensive vacuuming should the soil be too dirty.

I can understand you managing the same or bigger sized tanks successfully, even on 3times turnover, you definitely seem to have the knowledge and skill for it. I would like to get there as well, but for now I would basically do anything to limit to chance of algae.

At the end of the day, you need to go with what your comfortable with. There's nothing wrong with high turnover as long as your not blowing stuff all around, it's just not necessary IMO. As I said, plants and water changes are much more effective in keeping a tank clean than a filter. High turnover does not prevent algae in the same way plants and water changes do. Plants and water changes remove the effects of waste, filters just move it out of the main tank, but it's still part of the system until you change the water.

Can't help you on the fan, never used one, but I'm sure someone else will chime in who has.