Before considering "treatments" (which can often have side effects on the fish or bacteria or plants), you need to find the cause; otherwise algae and cyanobacteria will often return.
Algae is caused by light. However, what you describe as the slime coating gravel, etc. is not true algae but cyanobacteria. This is caused by organics in the presence of light.
You have a 180 litre tank (approx 50 gallons for those of us who "think" Imperial
) with two fluorescent tubes. Java Fern is the only plant you say; this will tolerate fairly low light, and is slow growing, which means it is not going to assimilate a lot of nutrients (from organics). Plus, as it is not rooted in the substrate like some plants, the detritus building up in the substrate is not getting used and the resulting organics leech into the water column. Overfeeding can also contribute, so have a look at that. Plus, the fish load; how many fish and what are they?
The light is adequate for Java Fern, but other plants would have a struggle. Excess light will feed algae. How long is it on each day? Is there much room light (from windows, etc.)?
Knowing the above will help us with the solution.
And, welcome to Tropical Fish Keeping.