Hi Darrel - though I think I've read most of what you've posted about the duckweed index, I wonder if I've missed some info where you've possibly commented on how one might interpret different 'symptoms' as seen with Limnobium ?
For example in one of my tanks the majority of the Limnobium leaves are a good strong green but I seem to be forever picking out single leaves that have somehow detached from the main plant. These themselves can be a good shade of green, but over time turn yellow and eventually melt to a brown sludge - while other intact plants are obviously thriving. It's possible that this is just mechanical damage as the frogbit gets moved about by fish and the filter, though the latter is a mild as I can make it, but for all I know it could be a classic indicator of something I'm unaware of.
Also to what extent is the yellowing of the older leaves that are gradually covered over and even submerged by newer growth an indication of anything, and to what extent is it just a natural process?
Any other tips on how to interpret clues from Limnobium? Other than colour and growth - for e.g. in some of my tanks the frogbit seems to grow much bigger leaves, as to making many more but smaller leaves? Some grow magnificent "bushy" roots while others put out tremendously long thin roots?
I think this is where EI comes into effect, it eliminates the guess work out of fertilisation. If you suspect that there are some issues just dose full EI for a month or so and see if anything changes. It's part of the elimination process. I think combining the two methods EI/DWI is the two most useful tools we have at our disposal better than any tests or DC'S which are at best a rough idea. It enables us to actually use the plants which are far better indicators.
In the case of DWI you can narrow down to fert issues and EI narrow down to co2 flow issues so outside of tank cleanliness you can gauge where things are going wrong. I suppose we have to look at EI as a bit of an idiots guide to growing plants so shouldn't be made complicated as it was designed to simplify things.
I do understand the EI calcs because it gives people something tangible, especially when we start getting down to sub 100 ltr tanks and dealing with 1/32 of a tspoon of po4.
However.like mentioned, chances are the vast majority of us are not going to need the doses with our particular lighting so it comes down to using the two techniques to try and work out how much we do need...if you want to. In my particular case I try and keep my TDS down a bit for the sake of the fauna and inevitably with full EI dosing that will rise because you are never going to be using it. I just counter this with a smaller dose now and again or even better an extra WC.
Coming back to plants, sometimes plants just get damaged or grow an odd shaped leaf now and again, it happens, just cut it off if it offends you, often leaves will get damaged doing wc's and maintenance, maybe snapping a stem which then dissolves and everyone's first panic reaction is omg not enough potassium or whatever. I've looked at a few images of real water courses with plants in looking for inspiration for shapes and you'll be shocked at how many wild species have holes in leaves or brown tips and these plants chose to live there, no point asking God if he can take a look at the Andes and see if there's enough potassium coming down there.
I think it's best to look at plants in the main, you maybe have the odd couple exhibiting something but in the main what are they like. I get certain leaves on Duck weed that aren't optimum but in general they are doing fine so therefore I assume my dosing is fine. Having very soft water and adding some iron root tabs as well as traces iron or chlororis in new leaves is rarely a problem although there are a few new leaves that are a bit pale. Old leaves tend to be more problematic with me holding back a bit of Magnesium for tds sake. If all other plants seem fine then I carry on as normal, and signs anywhere else I chuck in another quarter teaspoon of Epsom salts. I tend to find if DW is low on nitrogen they get lines on the leaves but rarely happens with dosing about 2/3rd of EI values.
As for co2, I also have other test plants so I combine the DW with this to get an overall picture and like Darrel I look for growth, if I'm seeing growth then things in the main are doing ok.
Some examples would be, if I take a look in my tank mid week, firstly the duck weed..
Few lighter leaves here and there, most of the yellowing ones were removed at weekend but in the main they're OK so no need to panic.
Then there's the Java fern there, notice the dark green tips, a sign of new growth.
Apologies for not knowing what the other plant is (I'm useless with plant names) but that is put there deliberately. It's been put in a place I would suspect of having the worst co2. Usually once a week, maybe two I would trim it down to about an inch off substrate level. In a week I would hope for this thing growing an inch or two. Same plant gratuitous bobbling shot haha
My MC carpet I look for new growth on the tops, yes there's a bit of algae in there but it's not taking over so no need to be looking for remedies just yet, it's just part of keeping a tank in my eyes.
I guess what I'm saying is people seem to be looking for problems for the least thing and in a healthy aquarium you are going to get a bit of algae and the odd poor looking plant, doesn't always mean you are doing something wrong. We're trying to create a water body scene and you will find all those things in there, it's not binary right or wrong, there's plenty in between just like EI dosing.
As long as things are growing I tend to leave well alone and concentrate on cleaning.
Just thought I'd put a picture of my GBR in as well for good measure. I couldn't get a picture of any of the plants there without them getting their mugs in so obviously Naomi Campbell and Tyson Beckford wanted to be part of the shoot haha
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