Thank you Commie for your advice!
Actually I have a big Mitsubishi 24,000 BTU Commercial AC so the fans I bought, the P12 PWM PST CO
https://www.arctic.de/media/05/64/2f/1583824874/spec_sheet_P12_PWM_PST_CO_190328_r6_EN.pdf
are good just to move enough air, as you can see the temperature of my GPUs before:
and after
Glad it works for you. However, these profits aren't going to last forever and using AC to cool your rig might send you into negative profit numbers
Hopefully my electricity cost is 0,07$/kWh and my AC has an automatic energy-saving operation i.e. the room temperature is automatically adjusted in order to maintain a fixed effective temperature, cooling operation is performed a few degrees warmer than the set room temperature once the temperature is reached.
I have difficulty to envision negative profit numbers. Let's simply take for example three 480 cards, from the website whattomine.com, mining ETH, it generates $12.51 in revenue per day (in date of May 15th 2:22 P.M. Easter Time) and with my 0,07$/kWh, it costs 0.71$ of electricity per day. I calculated my AC cost me in average 4$ per day.
So in this hypothetical example, the total electricity cost from the mining rig + A.C. = $4.71/day or
37% of the revenu goes to pay the electricity in this example but in reality I have much more than 3 GPUs so my actual electricity cost percentage which I monitor and calculate with my Power Distribution Units is around 5% of my total revenue.
There will be EIP-1559 in July which could impact negatively ETH mining revenues (-30%) and then next year ETH 2.0 will make it impossible to mine ETH but even when I look at the second most profitable coin to mine with GPUs, my electricity cost percentage will still remain very low.
I agree there's actually a frenzy around crypto mining but it is a really profitable business!