Bolly Olly wrote:

Think I'll have to buy some testing kit (any suggestions). I have a
90L tank with one fish & change 20L each week. Have 2x Fluval 3 plus
filters & 1x vigorous airstone. Also I hoover gravel about once a
month (this routine has gone on for 4 years).
Too be honest I thought I was quite good at house-keeping - obviously
not

Your house keeping seems fine. Test kits are recommended. Ammonia, Nitrite,
Nitrate and PH testers.