Ok, well, it isn't your pH that's the problem - plants can tolerate a wide range of pH, so if it's not causing your fish any problems it won't be causing your plants any problems.
Plants photosynthesize to make the food they need. For that they need three things - light, fertilizers and a source of carbon. 40% of a plants mass is actually carbon, so they need a lot of it! Land plants take in the carbon from carbon dioxide in the air, but there is much less carbon dioxide in water, so plants can't always take up what they need.
Light is what drives photosynthesis - the more light, the harder the plant photosynthesises. But, without adequate fertilisers and carbon, it's like trying to rev an engine without oil - it will cause damage and eventually kill it.
If you have a 55 watts over a 190l (42g) then you have 1.3 watts per gallon (WPG). This is classed as fairly low light - anything above 2WPG is classed as high light. So you aren't driving your plants to photosynthesize too hard. Unfortunately, it sounds like there's not enough carbon in the tank to enable the process to take place so the plants are dying.
There are two ways to add carbon - either with a yeast based system (you put yeast, sugar and water in a bottle, the yeast ferments and produces carbon dioxide which is then directed into the tank) - it's cheap, easy to maintain, but dosen't produce a lot of carbon dioxide, especially for such a large tank, although I would personally recommend it as a starter, you can always upgrade at a later date.
The second is to buy a source of bottled carbon dioxide (normally a fire extinguisher) and direct it into the tank - it is expensive to buy the equipment for this, although it gives good results.
Moving onto fertilisers, they can be classified into two types - macro and micro. Plants need both to photosynthesise. API Leaf Zone only contains micro fertilizers. Tropica Plant Nutrition + is a good fertilizer since it contains both. Alternatively, it is extremely easy to buy the chemicals and make fertilizer yourself - it's also much much cheaper than any commercially produced product.
i am a newbie to all this so you'll have to bare with me with my ignorance!!
We all started somewhere (and I still don't think I'll ever arrive at the station
) so keep asking questions.