One problem I never seem to get away from is finding a way to get good CO2 concentration all over the tank
. I can and do set up a Koralia powerhead to give me a circular water circulation all around the tank, but as the plants grow, that circulation gets slower and slower. So, I am tired of fighting that battle.
It has occurred to me that, in nature, most plants get their CO2 from the substrate, so ideally we should inject our CO2 under our substrate, but my idea of using silicone tubing leaking CO2 into the substrate didn't work out. Tom Barr suggested using a RFUG, reverse flow undergravel filter, instead. Lots of time spent researching the internet to see if anyone has succeeded in using a RFUG to distribute CO2 returned nothing. But, I did find that Tom was extremely enthusiastic about them back in 1999. So, I decided to give it a try this week.
The goal for a RFUG is to distribute water uniformly under all of the substrate, to use the whole bottom of the tank as a gravel filter. And, if you first filter the water with a canister filter, the substrate can act as a "polishing" filter only, but if that canister filter water is also passed through an external CO2 reactor first, the evenly distributed water will evenly distribute CO2, right at "ground level" for all of the plants, solving the problem of how to reliably get CO2 to all of the plants, all of the time.
Tom had a couple of other suggestions, dating back to 1999, make your own filter grid from PVC piping, and put the water exit holes in that piping under the tubes, so substrate material can't flow down into the tubes. That is what I am doing.
My design criteria were:
Minimize the canister filter flow reduction caused by the RFUG.
Achieve uniform flow from all water exit holes without having to adjust hole sizes and spacing.
Keep the cost as low as possible.
Tom prefers to use CPVC piping because it comes in smaller diameters, reducing the room the filter tubes occupy under the substrate. I chose to use standard PVC because of the ease of buying all kinds of fittings for it.
I designed this as an under gravel manifold, which means minimal flow velocity in the tubes for even distribution of the flow to all holes. And, I made use of standard fittings for every part of the RFUG. The finished product, before gluing any connections or painting the stand pipe black, looks like:
and the water exit holes are shown here - 7/64 inch holes on approximately a square grid pattern.
All of the piping sizes and the hole sizes and number of holes are designed to reduce the effect on the flow from the canister filter and give uniform flow from all holes. To check whether this works right I stuck a garden hose on it and with the holes pointing up, I compared the height of the fountain of water from all of the holes. It was almost exactly the same for all holes.
There is no need to glue any of the pipe fitting joints, since there is very little pressure on them, and PVC fits tightly into the fittings anyway. But, I will glue those connections that could cause water to flow all over the floor if they slip loose. And, to minimize the visual disturbance from the standpipe I will paint it black with Krylon Fusion. The brass valve is sold as a drain valve at Home Depot - it will help bleed the air from the unit when first set up and every time the canister filter is cleaned.
I hope to set this up by the middle of this week.