How to make plants thrive in my 40gallon?

[Luis]

Hello I’ve had my planted 40 gallon for a while now. 
the plants for the past months have shown deficiencies of potassium, iron etc.

so I took it upon myself recently to buy some Tropica Plant Care Fertiliser as well as seachem potassium. 
 

ive dosed the recommended dose for the past 5 days.

I just wanted to know how long should i expect it to take for the fertilisers to take effect? 
 

Any valuable links with HQ info would also be appreciated. 
thanks

[Yanni]

I think that it usually takes a couple of weeks for fertilizers to actually start doing anything. In my experience, it takes about 3 weeks to start noticing any affects. It different for many people but it mainly depends on what plants and how much you are dosing. 

[Andy's Fish Den]

Whenever I make any changes in my planted tanks, lighting, ferts, move plants, I always give it two to three weeks before I make any more changes. I have found usually by then, you will start to see any issues or improvement

[MJV Aquatics]

Although I agree with the previous posts regarding 2-3 weeks to notice effects, you don't mention your plant types. Ferts in the water column are generally good for all plants, but any rooted plants get 85% or more of their nutrients through the roots and may need a few well placed root tabs for best growth and health.

[Luis]

Jungle Val, Amazon Swords, Vallis, Java fern are the most abundant. 
athe Amazon swords specially are the ones taking a hit. The bigger leaves are brownish and like melted.

Any other tips you’d suggest? 
thanks to everyone’s responses