Soil question for the Walstad tank

My Pistia (water lettuce) is becoming increasingly pale for every week. Any suggestions for what the reason for this could be?

Edited on 1/28/23: I looked through my references and all the aquatic botanists use chelated iron (e.g., FeEDTA). Chelated iron will stay in solution longer than the non-chelated ferrous iron that you have, especially at that high pH. I vaguely remember reading that ferrous iron will get oxidized within minutes at pH 8 while it will last a few hours at pH 6. Chelated iron, in contrast, will stay in solution a few days once you add it to the tank.

Also, remember that DOC (dissolved organic carbon) also chelates iron. Some of my tanks never need iron addition, and others sometimes need it for floating plants.

FeSO4 may work, but you will need to add it more frequently and in tiny doses (make a stock solution with just a sprinkling--50 mg-- and use it immediately.). If you get a plant response (new leaves are greener) that will tell you if it is working. If it doesn't work, then get chelated iron powder or a trace element fertilizer. I wouldn't dump FeSO4 into the tank as it contains sulfates and you don't want to load up your tank with sulfates.

I wouldn't worry about the pH, especially with the guppy and snail additions. Overall, your tank is nice.

Have not seen a "plant response" yet with FeSo4, but my other plants look ok (see pictures).

I will buy another form of iron and see if that can help the floaters.





I am also thinking about finding the optimal gravel size as a cap for the soil. For the two cubic tanks (31L) I used 1-2 mm inert gravel, but for my 54L (60x30x30), I have used 3-5mm inert gravel as a cap (see picture). Can anaerobic pockets be reduced/minimized by using a larger gravel size, or is it only the amount of organic material that decides? I will try to find out...

I am not saying that I will tear the tanks down. I am just asking what the optimal gravel size is as a cap. By optimal, I mean the gravel size that leads to the lowest production of anaerobic pockets in the bottom layer/soil. Will the number of anaerobic pockets be different if I swoop 1-2mm gravel with e.g. 5-8mm gravel? Or is it only the content of the soil that decides production of anaerobic pockets? Anaerobic decomposition do not require oxygen, but uses nitrogen and phosphorus. If I then have a soil with minimal N, P and S (e.g. bark fines), what will then drive the decomposition? Anaerobic bacteria at a very low level I guess (?), and since the soil is low for N, P, S the production of biogas products like methane CH4 and carbon dioxide (CO2) will be very slow or? My four different test tanks (with different soils) have now been filled with one inch 1-2mm cap and water for about 10-11 weeks, three of them produce air bubbles, while the one composed of peat moss and bark fines do not. The three soils ("tiger", "oslo" and "sowing" soil, described in detail earlier in this post) that produce air bubbles are all rich for "nutritions", with added manure.

maico996 said:

I haven't gone back and read through this thread to see what inhabitants you have in your tank, but if shrimp and MTS are part of the equation then I'd go with the 1-2mm size gravel. Much easier for the MTS to burrow in and the shrimp will have an easier time picking up smaller pieces of gravel as they go about their shrimp business. As JohnWesley has pointed out, rooted plants will help prevent anerobic soil conditions, as will MTS. I'm pretty sure Diana refers many times to the role plant roots play in oxygenating the soil. Regarding the depth of the cap, whatever size gravel you decide on, 1 inch is the maximum recommended. You could use less if you want...its main role is to keep the soil from getting into the water column.

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I have guppies and nerite zebra snails right now, but I will try to add some shrimps too, So, yes, I will keep at least one tank with gravel size of 1-2mm, even though I probably will get a lot of anaerobic pockets after a while. I have already teared the tank down once. I removed 75% of the soil and re-mixed the soil that was left with 50% 1-2mm gravel before I put a 2cm cap on (with 1-2mm gravel). So the bottom layer is now about 3-4 cm deep with 1-1,5cm soil and 2-2,5cm cap. Lack of solid roots systems in the bottom layer will probably ruin the tank after a while. So, it is best to enjoy the tank as much as possible before everything starts to smell again



Have a great weekend!

Plant roots can can keep a soil substrate healthy and not smelly. Unplanted areas should not have a soil underlayer.
In this picture of my tank setup notice how I constructed a front section rimmed with rocks and containing a thin layer of sand with no soil underneath.

dwalstad said:

This looks like a classic symptom of iron deficiency--yellowing of the youngest leaves. Iron is the one nutrient that can hold back floating plants that depend on water iron. (Rooted plants can get iron from the soil.) Iron is the one nutrient that is often deficient in the water, because it forms iron oxides and precipitates out making it unavailable to plants and algae.

I purchased FeEDTA powder. I quickly make a fresh stock solution of a pinch (200 mg?) in a cup of water and then add this cup to every 10 gallons of tank water. Doesn't have to be exact. I might add it just once or twice a month.

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Looks like you was right, after I added some "fresh" iron to the tank my floating plants seem to produce fresh green leaves again.