Huge pH swings (7-9 during a 24 hour period) indicate that your water is deficient in bicarbonates (as measured with a low KH of <2). Since many plants (about 50% of submerged plant species) can use bicarbonates as their carbon source, a low KH is not ideal for a natural tank (i.e., no CO2 injection). Bicarbonates both prevent diel pH swings and provide much-needed carbon for many aquatic plants. I would add baking soda (1 teaspoon/gal) to get KH up to 4-6 degrees.
I have low KH now but in the past is was higher and I still had PH swings. The reason is simple. most of the KH in water is calcium carbonate or magnesium carbonate. Yes plants can consume the carbonate, but when they do that leaves behind the calcium and magnesium. These covert to hydroxides and push PH up. Also when plants are growing they consume nitrates converting potassium nitrate to potassium hydroxide which also pushes PH up. hydroxides have a much stronger effect on PH than carbonates. Over night the hydroxides latch onto CO2 other compounds in the water and stop pushing the PH allowing it to drop does back down to 7.
based on basic chemistry KH doesn't prevent the PH from going up it only prevents it from going down. Also I now have CO2 and still see the swings. The best way i found to reduce the swings is to slow plant growth by dimming the light. When you have acids in the tank they react with the carbonates forming salts that have no effect on PH. Using a sea shell works much better in controlling PH. If the PH is 7 or higher than 7 nothing happens. the solid KH of sea shell stays solid and won't affect the PH of the water. however if the water turns acidic some of the solid carbonate dissolves neutralizes the acids allowing the water to go back to a PH of 7. This is how it works in nature since soils and rocks have calcium carbonate that will react with acids.
Adding baking soda is also not a good idea. That adds sodium pushing up the KH to 4 to 6 with RO water means the water is almost entirely sodium. If you have naturally soft tap water you also have the same problem. in nature fish live in water with a little bit of sodium and a little bit of potassium. If you put fish in water with just sodium they will eventually die. If you put fish in water with just potassium again the fish will die. And it is not only fish. Excess potassium or excess sodium kills people every day. Its a common issue with any animals. and evidence of it occationally shows up in forums:
Snail troubles - SOLVED
besides in nature you rarely find sodium bicarbonate or potassium bicarbonate in river or lake water. Most of the time the you find sodium and potassium sulfate, halide, borate or molybdate salts instead. But you frequently find Calcium and magensium carbonates. If you want to be as natural as possible don't add sodium and potassium carbonates to aquariums. its not natural and it is not safe for fish.
You have supported my recommendation for adding chelated iron fertilizers at night or just before lights go out. That's when light and a potentially higher pH would be less likely to degrade the chelator.
If you look at all iron products out their you will find all are chelates but not all chelates are equal. The chelated irons are Gluconate, EDTA, DTPA, and EDDHA. Iron gluconate is rapidly attached by bacteria causing the iron to rust and can cloud water. It lasts about a day but it is cheap. iron EDTA oxides when the PH is above 6.5. It can also cloud the water. I recommended Iron DTPA it is stable up to 7.5 and sometimes up to a PH of 8. Above that it cn oxide and cloud the water but most tanks are below 8. Iron EDDHA looks very promising. Stable up to a PH of 10! However there is a catch. It colors the water red. So of all of the Chelates available the most commonly used ones (Gluconate and EDTA) can cloud the water because they are not optimal for most tanks. UV light does degrade chelates but regular indoor or artificial lights don't have UV so aquarium lights won't degrade the Chelates.
it shouldn't take much (100-200 mg chelated Fe/10 gal tank). To a 10 gal tank, I add a powdered quantity of FeEDTA that approximates half the size of a green pea. Does not have to be exact, but remember that iron is required in small amounts. It can be toxic to plants and stimulate algae at excessive doses.
100 to 200 mg in a 10 gallon is 2.5ppm to 5ppm. based on some information I have seen on line the toxic limit for fish is around 4ppm but PH and oxygen levels affects this. I use only dose to 0.1ppm once a week in my tank and don't have any iron issues. my dose is based on the amount of iron ion plant tissue. Your dose is also the recommendation for iron gluconate. Neither gluconate or EDTA lasts long in most aquariums so you need to dose a lot more and can push it right up to the toxic limit to get plant growth. I use DTPA which is comparable with my PH so I need very little.
Also when the chelate breaks down the iron will oxidize or react with something else in the tank. And that often is Phosphate. Forming Iron phosphate which is not soluble. So if you use the wrong iron you can end up with a phosphate deficiency. Even though you are dosing more than needed.
