plants fertilizer

Hi. I'm quite new to the hobby. I have a fluval flex 57 litre tank setup. I have had live plant in my aquarium since I decorated it from day one which was about 8 months ago but now the plants look like they might be struggling abit. I'm guessing the plants have used all the nutrients that were...

Does anyone know of any liquid fertilizer that has minimal effect on the TDS? I am currently using a brand called ISTA fertilizer but it seems to increase the TDS quite high. Can someone recommend any other brand? Thanks in advance.