AndEEss

I think you'd need just a single FX4 for a 75g tank. Just make sure that you perform monthly maintenance on it (take the foam out, slap it on a rock, return to filter) so the flow rate doesn't drop down with detritus accumulation. I don't run an air pump. There's a good amount of surface agitation and the many fish I have in there seem to be doing just fine.

It's not the amount of water the filter can hold, but the volume/surface area of the media it uses. Additionally, from a FILTRATION perspective, flow rate isn't super important. However, from an oxygenation perspective, flow rate is important as it will greatly influence surface agitation. My Waterbox 6025 (60.5 x 25.3 x 21.7", 143.8gal) has a FX6 and FX4 running in it. However, those filters/pumps also drive a DIY inline heater, a UV filter and a Cerges reactor. So, while on paper I get about 1000gph (450gph + 563gph) of turnover, I'm guessing it's closer to 900gph with all of the hoses and hard bends in the equipment. Still, with ~120 gallons of water in there, that's way more than enough to satisfy both filtration and oxygenation requirements.

A few drops of ammonia in a 20 gallon tank is not enough to properly cycle it. When I cycle a new filter, I calculate AND test to confirm that 2ppm ammonia have been added to the tank. Also, when you use liquid ammonium chloride, it can cause a drop in pH over time, as it depletes KH. So you'll have to add a bit of something (baking soda, etc) to increase KH to keep your pH up above 6 or so.

Stir up water, perform water change. Stir up water, perform water change. Stir up water, perform water change.

Can you name the specific pieces of equipment you're using, or provide any pictures of your setup? I can't imagine having any water-borne algae with an appropriately sized UV sterilizer.

Pour in sand, do repeated partial water changes with dechlorinated, temperature-matched water. Don't overthink this.

Bladder snails and pond snails don't eat healthy vegetation, in my experience.

Swords are typically grown emersed (leaves partially or fully above water) at the large, industrial scale warehouses they come from. Why? Less/no algae, greater access to CO2, etc. As a result, the mature leaves that grew above water melt back. New leaves that are 100% submersed will replace them. I would trim those leaves off so the plants can put more energy into growing the new leaves.

For starters, if it were me, I would look up the optimal water parameters of all of those fish and determine any incompatibility issues, if any. Turns out, the fish listed like very different water.

Make your own: https://www.plantedtank.net/threads/le-dukes-wb6025-journal.1327533/post-11480328

You saw nitrite because the tank wasn't cycled.

Water level should be high enough that you get surface agitation and disturbance across the top of the water. The water plunging vertically into the water is NOT what you want for optimal oxygenation.

6 pairs of rams in a tank with less surface area than a 20 gallon? Not the route I’d take.

Nitrates aren’t the only thing; all of your flora and fauna are producing dissolved organic compounds as part of their normal metabolic processes. In addition to traditional macronutrient imbalances, these are often the causes of algae problems. Particularly BBA.

I do a 60% water change once a week on my 140. Fill up a 275g plastic container, and water our plants with that.

Again: the fish you plan to keep are NOT compatible with each other.

To paraphrase JP Morgan, “If you have to ask, the answer is no.” Seriously. Discus require temperatures that are going to really, really rough on the listed fish you currently own. And, 75g is a pretty small tank for 6 fish that will get to be 6”+ in diameter. I have a 5x2x2 and think that’s too small for them.

Is there water in the inlet and outlet hose?

Did you properly prime it before starting it back up?

UNS Controsoil hasn’t released any detectable ammonia for me. Used 20+ liters in the planted areas of my 140g tank.

So, it appears you have some kind of small-grained gravel or even an aquasoil. Both of those are going to limit the effectiveness of an under-gravel filter.

Black neon tetras. School/shoal well. All 10 are together, 99% of the time.

So can a canister if you do it right. I have nothing besides inlet and outlet pipes in my big tank. Everything else is plumbed in-line from the FX6 and FX4. CO2 reactor, UV, heater, dosing pump all in the cabinet.

There’s nothing wrong with your pH. It’s a good pH for growing plants. How old is the tank? How did you cycle it? Is your brown algae diatoms?

Your tank isn’t cycled because you didn’t add enough ammonia to feed it. Unless you get significant ammonia, as in 1ppm+, you aren’t adding enough to get detectable nitrate at the end. Additionally, you have to continually feed your BB.