Newbie needs help, temps and possible Ph issue?

Ok so complete newbie here, i have researched and researched some more but need definitive answers from someone who knows what they are doing.

Growing in a loft tent, 600w metal halide, have a 5" intake fan coming in from the bottom and 5" carbon filter and fan going out at the top, temps are reaching 90 degrees at times, don't want to leave the tent door open due to dust or bugs, how can i reduce these temps? would removing the carbon filter help as we dont really need this on anyway at the moment? i have an oscillating fan also, when lights go out they are dropping to around 60 degrees which i think is too low a drop, have a 2ft tubular heater in there and without this temps were dropping to near 50!

Plants are not growing very fast and leaves seem to be yellowing slightly, they are 3 weeks old and can see roots at the bottom of the pot, do these need repotting now, originally they were in coco but thinking when repot to add perlite 25% to help drainage, tested the Ph of water and nutes which is 6.2 and run off is 6 but when we have used the soil tester it states ph is 7? do we need to use Ph down next time we water?

Didnt think this would be so stressful but just want to get it right so i dont keep disturbing the plants all the time?

Any help would be appreciated

Thanks

Is that 90 degrees room temperature, or 90 degrees at the plant tops? Either way, it is way too high. The first thing I would do is check room temperature. No matter how good of an exhaust you have for the light, if the room temperature is too high, then it's all bad. Are you able to run A/C at all? You might want to invest in a small portable one for that area.

I try to keep my room temperature no higher than 70 degrees, though it's difficult at times. Have to figure it will be a few degrees hotter beneath the light where the plants are, and you want to stay out of the 80s.

Your exhaust fan may not be powerfull enough for you room. What size is the room, what kind of fan (cfm rating)? Can you maybe connect your intake to some ducting and put it near a window to get cooler air from outside?

coco doesn't need anything to help drainage, it is a hydroponic medium. ph the solution not the soil, set it to 5.8 then feed. don't bother testing your run off. what are you going to learn from it? make sure you calibrate your ph reader every week. yellowing happens when the PH is too high in a hydro grow. fix your ph and it should fix the yellowing. give it 5 days and you will see. give them a really really good soaking to flush out any previous solution, water all around the medium and really drown it out, you cannot over water coco. and roots at the bottom of the pot doesn't always indicate the need for repotting. stick your finger into the medium, and have a little feel around. if it is solid with root structure, then repot. or just pop it out of the pot and have a look if the plants are small. if it is tightly bound with roots, holding all the medium together, repot. if it is fairly loose and medium breaks apart, don't.

if the plants are 3 weeks old, they will grow slowly, they will catch up soon enough. the cold doesn't help them though.

as for temps, it doesnt help that you are in a loft. i suggest you invest is a twin temp fan controller and a thermostat for your heater. this way you will maintain a better stable temp range. you want around 20-21 degrees C at lights out, 25 with lights on. it might be too late now for you but i suggest you run your lights during the night and off during the day. this way your lights are getting cooled by the night time temperatures.

if you are running a roof tent, you are probably no bigger than 5'x5' so your fan is plenty adequate enough for that size. however you dont really want your intake being the same size, you want to create negative pressure. with your carbon filter slowing the flow in your exhaust, your intake is beating it, giving you positive pressure, which is bad.

happy growing

Thanks guys for your reply,

The tent is 1.2 x 1.2 x 1.8 high, the exhaust fan is an RVK 125 fan (not sure of the CFM) and the intake fan is a TT inline fan CFM is 164, the intake fan was originally used for the cool tube light but was getting too cold in there so just bought a reflector hood instead and now its too bloody hot!.

A bit of adjusting today and we have managed to get temps down to 82, added another clip on fan, took off the carbon filter, extracted warm air from tent into the bedroom via ceiling vent, intake we have managed to get enough ducting to the edge of the roof where there is a small opening, not enough though really but at least its something, there is a bigger hole but need more ducting to reach it, cant really afford any more gear as we have overspent on budget for now. Humidity is around 42% so i think thats ok?

Have decided to re-pot them, medium was firm when i took it out so roots were holding it all together, decided to add 25% perlite to see if that helps them, just hope iv done the right thing now.

Are temps ok at low 80's or do i still need to bring them down a bit?

We run lights from 4pm to 10am as to make use of the cold evenings, lights off in the day but its so cold at the moment???

Thanks