Ammonia and nitrites will usually not take long to show problems in your tank. They are lethal and will work quickly. Nitrates are a different story. Nitrates take a while to blossom into a problem. They will take months and even longer and then all of a sudden the tank crashes and you have all types of problems. I know you said you`ve had them for 8 months but without proper removal techniques or a lesser bioload your tankmates might be living on borrowed time. If you are going to keep the fish then I wouls suggest 25% PWC
`s every week. I know in my early days I used a denitrator that helped out alot.