sata cable not strong enough to power fans?

I have a 6 way fan block that is powered by a sata cable, however whenever I attempt to power on just the fans it seems like the fans are running at half speed, one will not even turn on, but yet if I use an adapter to go from molex to sata, everything runs full speed with no issues. the molex cable I have coming from my power supply has 4 ports on it and all 4 are technically being used, for pumps and what not, so id like to avoid running a splitter. I have 6 plug in options for 6pin on my psu and have tried all options, and it seems like sata won't fully power 6 fans, 3 fans or even 1 fan at full speed. Also the sata cable only has one port on it. Any ideas? Another thing the sata cable did not come with my psu, I got it on ebay, and swapped the 6 pin connection on the end as the one that came with it one of the plastic pin covers was square instead of the shape mine is. corsair ax1200w psu, and corsair sp120 fans, in case you needed that.

SATA and 4 pin molex pull from the same rail, so that shouldn't be the issue, unless it is just having trouble splitting all off of a single cable? I would try adding a second SATA cable to your modular PSU and see if they all power on if they are all on the same one apart from the pump/etc.

ok so im gonana try to make this clearer.

Only this is plugged in--- sata cable from psu- 6 or 3 or 1 fans all at half speed, nothing else hooked up to the sata cable
only this is plugged in ---psu to 4 way splitter molex cable- 1 swiftec pumps and leds for my resevoir, and fans everything works 100 percent,
if everything is plugged in to the psu, im still only getting half speed on the fans, the mobo is being jumped during all of this.
with everything running , mobo, graphics card processor and ect, rwill my fans run full speed?

are the fans not pulling enough power for my psu to fully power the cable? I have also attempted to plug in 8 fans to the sata, still all at half speed. is it possible for the cable to be bad? its brand new, no corrosion or split wires all good to go.

A single molex can carry 11 amps on the 12v line.
A SATA cable can carry 1.5 amps on each of the 3 12v contacts. Your fan block is probably only pulling from a single contact, thus you are overloading the cable.
Can you not power your stuff a different way?

I just dont have a second molex power cable so if i plug everything into the one molex power cable i have, Then im running alot way more than i should off just the molex cable. I guess im gonna have to attempt to find a new molex cable.

I'm confused, where are the rest of your cables going? The ax1200 has a ton of power cords

1 sata 6 pin to dvd drive,
1 sata 6pin to 6 fan block
1 sata 6pin to ssd/hhd
1 molex to 2 pumps, reservoir l.e.ds, and 4 fans.
space isnt the issue just lack of other options. no more cables.

Did you buy the Corsair AX1200 used and it just didn't come with all of the cables?

Have you tried swapping SATA power cables? For instance, swap the one from the optical drive with the one you had been using for the fan block. Make sure the one you had been using for the fan block doesn't have the wires out of order, like in a manufacturing snaffoo, such that you are getting the wrong voltage. I'm no expert on this but I do know you can swap the order of the wires on a molex plug and run a fan either on 12v, 7.5v, or 5v.