Lots Of Algae Forming

Another thing, the business of Corydoras barbel problems probably has more to do with keeping the substrate clean than anything else. Some people like sand because the mulm (fish poo) sits on top of the sand and is easily seen and siphoned off. However, I prefer gravel because the mulm goes down into spaces between the gravel particles and becomes plant fertiliser. That's probably why my plants grow strongly without adding fertiliser. I hardly ever vacuum the bottom of my tanks with gravel or sharp sand, the ones with finer sand need doing regularly.

I used garden soil. Others are wary of this in case of possible pollution e.g. by pesticides or weed killers, but unless your garden has been treated recently, I wouldn't worry about it. An alternative is John Innes compost, I suggest Number 1 or Seed type. Never use soilless/multipurpose compost.

The grit is harder to find as most garden centres don't stock it. The best is usually called alpine grit. Beware of J Arthur Bowers Coarse grit which is like the stuff they use on roads! If I were starting all my tanks again, I'd use J Arthur Bowers sharp sand (Horticultural, lime free). Sharp sand is a mix of sand and fine gravel, what happens over time is that the sand particles fall through the larger gravel and you end up with a layer of fine gravel on top. Garden centres are the places to look for all these. Having said all that, as you are talking about a fairly small tank then you might be just as well off getting aquarium gravel, but I'd recommend the finest grade you can get. Whatever you get, don't forget to wash it thoroughly in a bucket before you put it in the tank, to get rid of all the finest material which will cause cloudiness in the water. Keep washing until you get clear water.

About 4 cms of soil topped by a couple of cms of gravel should be about right, it's not critical but don't bury soil under a deep layer of sand if you use it, gravel can be deeper

The soil will be well covered by the gravel or sand and shouldn't be disturbed. You need to add water gradually, put a saucer in the tank, then pour water onto the saucer so that it overflows gently over the sides without disturbing the substrate too much. The first inch or two of water you add might be slightly cloudy from any remaining fine stuff in the gravel, if so siphon it off before adding more.

I find it easier to plant up early, when there is very little water in the tank, planting can be tricky if you have to have half your arm in water and the plants often float away as fast as you try to plant them. It's also easier to put plants exactly where you want them before you fill the tank. Some people go on about plants interfering with the ammonia cycle as they will use some of the ammonia themselves, I think this is another case of making a fuss about nothing, it has never given me any problems. Growing plants will also use some of the nitrates that are formed as a result of the ammonia process.