Thankyou so much for your reply Pat!
Yes I have measured the tank to double check and your are correct it is 190 litres.
I will continue with water changes and gravel vaccuming, I hope my fish can hang on in there until I lower the nitrate to a safe level, it's so hard to be patient with the low percentage water changes knowing that the nitrate level is doing them harm.
My silver dollars are approx 2-2.5inches from nose to end of tail (these were a recent addition by myself before I educated myself somewhat on the importance of testing water)
My pleco is around 8 inches although it's hard to get a precise measurement as he's very shy.
Does this still work out as my tank being overcrowded? If so, Would you recommend I take pleco to lfs and get an infant one once I've stabilised nitrate level? Or perhaps another bottom feeder? I've already become quite attached to him/her