T5 is very bright (intense) lighting. Without replacing the fixture, not much can be done about the intensity, so balancing it better is the option; if you can reduce the intensity somehow, that would probably help, but assuming not... Floating plants help with this. Ensuring sufficient nutrients.
The algae on the leaves is a form of black brush/beard, I have dealt with it once or twice, and also the bushier form which is more common. Light and nutrient balance is the issue. Light is pretty much set as I mentioned above, so let's look at nutrients. There are signs of deficiency in the swords. Substrate tabs will help this, I recommend Seachem's
Flourish Tabs as the best I have tried, but there are undoubtedly some good brands in the UK.
As for liquid, you need a comprehensive product. This means all nutrients are present, and in balance to each other in the liquid. Seachem's
Flourish Comprehensive Supplement for the Planted Aquarium that seangee mentioned is a very good liquid, I use this too. There is a product in the UK that should be basically identical,
TNC Lite, here is a link I found:
TNClite is Nitrate and Phosphate free. It is specifically designed for use in lightly planted aquariums and for aquariums with a high fish
www.aquariumgardens.co.uk
Do not use the
TNC Complete, it has nitrate and phosphate and you never want to be adding these if you can help it. The TNC Lite is or should be perfect.
The algae now present will not go away; the aim is to find the balance and then it will not increase, and that indicates success. In time as new leaves appear the older ones with the algae can be removed. It can take a week or two or three before you see improvements, so have patience. Reducing the light by above methods and improving the fertilizers will likely do it.
I also agree, do
not use liquid carbon. This toxic substance is highly dangerous to plants, fish and bacteria.
Last issue is the high nitrate in the source water...that is a significant [problem as it is weakening the fish slowly. Nitrates in the source water have to be filtered out before the water is added to the aquarium, as opposed to filtering in the aquarium. Nitrates occurring within the system is one thing, and relatively easy to deal with, but nitrate in the source water needs special attention.
@seangee and
@AbbeysDad have dealt with this and can better advise.