Refused to replace product under guarantee

On 13 November 2014 I bought a DoPhin F800 pump for my fish tank. After 3 months it started giving problems aerating the tank. I took it back to the shop where I bought it. They refused to exchange the product but offered to send it away. I explained that the tank will be without a pump during that time. They could not give me another option and refused to give me a phone number for the owner of the shop or any more senior person I can talk to. The lady in charge of the shop had a very poor attitude and tried to turn the tables on me by telling me I never cleaned the filter and that I waited too long before returning it. I will be obliged to buy a new pump while I wait for the faulty pump to be sorted out. I am not prepared to do so. I already lost a fish and another one seems to be going the same way. I have been a happy customer for years and although I have a pet shop around the corner from where I stay, I always preferred Tropicum. There has been a change in the management of the shop and ever since the service has deteriorated.