Filter Tinkering - Musings

@fcmf

I made it to the end! Some thoughts...

You know your individual fish so much better than me, and I suspect many others here too. The age of your harlies is a major consideration.  With age comes susceptibility to all sorts of things, in the wild this can often mean that animals meet an earlier demise than in the more controlled home aquarium environment. The s shape of the spine is generally thought of the be a genetics problem, I'm not sure that flow would have much of an impact on spine shape? Age might.  From some brief research; Harlequins are from naturally very slow flowing (if at all)  environments. Neon green rasbora in contrast like a bit of flow, though I also read that they need more oxygen than most. I wonder if this is a factor here too?...

You mention the co2 side of things... I can probably be a little more 'scientific' in response here... and make a big loop back to my oxygen question... no idea if I'm on the right tracks with this re the more fish related part of your query (others are better placed to validate this bit) but it is hopefully useful nevertheless for the plant side of things.

Firstly whatever you do, your are growing a plant in a co2 poor environment. This is why in 'low energy ie non co2 injected environments, plants grow slowly (note I didn't say unhealthily). Those who inject co2 often complete a pH profile to understand if they have injected enough and how it's being used over the course of the day. They are looking for a 1 pH drop before lights on and it to stay that way until lights off.  You are not going to see anything like this level of co2 in your aquarium.

There will be two sources of co2 in your tank fish/bacteria respiration (also reduces oxygen), and atmospheric. Diana Walstad recommends low flow and a siesta lighting schedule (which I also discuss on my site but am looking to remove it shortly due to reasons below) this is to preserve any co2 in the tank and allow it to build up in the siesta.  Diana's tanks are all dirted tanks and the high amount of bacteria respiration in the substrate creates co2.  This theory is now getting applied incorrectly by many (myself included) to non-dirted tanks such as yours and mine. In these situations we want to maximise on the biggest source of co2 available to us. This is neither fish or bacteria respiration it is the atmosphere itself.
Aquarium plant growers know this better than us mere hobbyists and co2 enrich their greenhouses and grow plants in shallow tanks so the co2 can reach the plants.

Co2 dissolves quite readily in water to a low concentration but also gases off quite readily beyond this low value of about 15 times less than atmospheric level (unless the atmosphere is co2 enriched as above). Flow, if it creates a ripple on the surface, increases the surface area for co2 to be passed one way or the other from tank to atmosphere or from atmosphere to tank. This is how airstones work, they don't add much oxygen through dissolving the bubbles themselves, they increase the surface area of the water to atmosphere and this is what does the job.   

So whatever your filter is supposed to do to co2 levels with whichever spray bar... id ignore the manufacturers advise here unless they specify whether they are applying it to a low or high energy (co2 enriched) aquarium. Given the knowledge above you can see that the best option for you is to maintain high surface agitation. This will increase atmospheric to tank co2 transfer, the biggest and best source of co2 for you, helping maintain a steady level of co2 in your tank.  This is also the aim in high tech aquariums, a steady co2 level during the lighrting period.  High tech aquascapers also maintain high flow levels with lots of surface agitation to maintain higher oxygen levels too to support fish health in this co2 laiden environment. So surface agitation in low tech acts to increase co2 and oxygen levels (ref earlier query about the neon green rasboras) so win win for you and I!

Don't forget also that floating plants have access to atmospheric co2. As such they don't deplete it from the waters column. They will get nutrients from the water and suck they a faster than your sword plant as they can access better co2 levels.  They are therefore are a useful indicator of poor nutrient levels in your tank which affect leaf health on your floaters first as they grow faster. If you notice any deficiencies here you know you need to start dosing ferts.  Conversely they don't like having their leaves wet so ensure that whatever you do with your filter flow and surface agitation doesn't impact them in this way.

So now to loop back to oxygen... not sure on your current filter setup 're flow and surface agitation, but... it's probably not making a huge difference to the plants (co2) unless extreme (slashing the floaters or over say 15x tank volume turnover per hour and tearing up the plants). You do have a lot of oxygen users in the tank (fishes) so its probably more important to do what you can to promise good oxygenation of the water over and above anything plant related.

Oh dear I seem to have added my own ramblings... I'll stop now... did that help at all? It's made be realise how much I need to update my site!...

Great clip. Everyone seems to be pottering around the tank quite happily. Well done.