Okay I have 2 concerns:
1. 38 litres = 10 gallons. This is a bit small for 2 dwarf gourami's and for both of the plecos, though I know now you are down to 1. Gouramis are somewhat aggressive and territorial and in 10 gallons they will not have much room to play out this aggression without causing harm. So I would expect that separating them is your best answer and hopefully this will help the one being bullied.
2. While your nitrates are within safe levels I do find it suspicious that the levels rise so quickly. Part of this has to do with the size of your tank. Small tanks are hard to keep stable. With the plecos that were in there I suspect that your filter cannot keep up with the bioload. Plecos are very messy and even a single one would be able to overload a small tank, especially if it has a filter that is rated only for 10 gallons as well.
One long term solution for this tank would be to buy a filter that is rated for at least 30 gallon tanks (if you do not already have one, that is!)
Solution #2 put the pleco in an appropriate sized tank: For a bristlenose this is 20 gallons as they do tend to stay around 5 inches. It is not horrible for the BN to be in the 10 gallon tank, but if you choose to keep him in there then I would definitely get a better filter to help with his messiness.
Solution #3 Make sure you do not have any more than one dwarf gourami in this tank at a time.
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