Bba Control And Fertilizers

Hi,
I have an almost cubic tank, 45cm x 45cm x (H) 48cm, gross volume 90 liters, actual volume more or less 75-80 liters.
I started the tank as a "Walstad method", low light tank in 2015. In 2018-2019 soil started to seems depleted, and I added some fertilizers tabs (Dennerle) to add Cryptocoryne, that restarted to grow again. I was also dosing Seachem Trace, 1 ml every two weeks, and a bit of iron.
Tank was started with garden soil topped with black quartz gravel.

Last summer I rescaped a bit, mainly because a probable lack of nutrients and a very warm period stunted so much the plant growt that I lost some fast plants, like Limnobium and Riccia fluitans.
In October 2019 I added Cryptocoryne crispatula, Marsilea hirsuta, Bucephalandra "Lavandau Green" and "Apple leave", and Lemna minor and a Pothos with roots in the tank as fast growing plants. Few weeks ago I added also a bunch of Bacopa caroliniana.
Other plants that I have are:
Cryptocoryne parva
Cryptocoryne beckettii
Cryptocoryne crispatula
Anubias barteri var. "bonsai"
Anubias barteri var. “pangoline”
Monosolenium tenerum
Vesicularia dubyana ("Java moss")
Microsorum pteropus “narrow”.

Animals are guppy-Endler hybrids, Amano shrimps, and snails.

C. beckettii, C. parva Anubias and Monosolenium I have from an older 30 liters tank (a shrimp tank) from 2012,

I am using a 15W LED lamp that surprisingly allows Marsilea and C. parva to carpet the bottom of the tank, and also the Bucephalandra is growing very well on a stone.

I am using tap water, that is very hard: KH=10, GH=20, but I have grown several species of plants in this water without problems. Cryptocoryne, Anubias, mosses, Egeria densa, Najas guadalupensis, Lemna, Limnobium, Riccia fluitans did very well, provided there are enough nutrients.

Some months ago BBA started spreading, altough slowly, but now are becoming a problem.
Lemna minor and Pothos are growing very slowly, so there is a clear lack of nutrients. I had Limnobium laevigatum for years, but slowly disappeared.

I tested the water, it has NO3 = 0 mg/l and PO4 = 6 mg/l.
I have ordered some root tabs for Cryptocoryne, and I have collected some ferts, but I would like to have and advice about how to dose.

I have:
Seachem Trace
Seachem Iron
KNO3
KH2PO4

I think that I shall not dose KH2PO4 for now, since I already have enough phosphates, and instead I shall start dosing KNO3 and see what will happen. Fishload is really small.
I am not dosing Iron either, since I noticed that if I add 1 ml of it, water start to be "foggy": probably because of hardness and phosphates are precipitating the iron?

Is this a good strategy? How much KNO3 shall I add?

 

I already did a good clean-up of some mulm from the botton and a 20% water change.
It is a low-light/no-CO2 tank, so I do not do weekly regular water changes but more monthly or every two-three months depending on the water colour: when the water starts to be too "tinted", I do a water change.
The filter is a simple sponge filter, I clean the sponge when I see that the output start to slow down (again, it happens about once per month).

From the article about low-tech, no-CO2 tanks I undestood that I shall dose a tiny amount of KNO3 weekly (1/4 or 1/8 of teaspoon for my tank, I suppose): is it correct?

 

Thank you, I planned to clean a bit more and remove a bit of vegetal debris. I will start dosing a bit more NO3 and see what happens.