What would cause my pool water to bypass the sand filter?? When i vacuum the algae off the bottom of the pool, i can see it coming back thru the return. The filter removes some of it, but not all. We already put new sand in the filter, and even put a whole new head on the filter thinking it was a problem with the gasket. (tried to change just the gasket, but had so much trouble we had to get a new head) :( What else could be the problem???

The cloudiness could be suspended particles or algae. Use an algae k i l l er as recommended, then a flocculant (water clarifier) as recommended, then vacuum the pool bottom and run the filter overnight.

After that check the ph, alkali level, chlorine level, and adjust as needed. That should do it.

your filter has removed the rust from the water backwash and use a filter cleaner when vacuuming a lot of rust off bottom or algae vac to waste so it doesnt go through your filter

I think Brandon's answer deserves a little more time and a slight explanation... as I think it is Spot On!
Algae dust is usually "Already DEAD" algae!
And normally it is way to Small & Fine to be trapped by the filter.
We here on this forum do not know what type of filter you have, But if you do have a "MultiPort" valve as Brandon suggested you turn to the "Waste" mode, you will not use the filter mode to vacuum, as this will only BLOW the Fine Silt like substance right back into your pool, spreading out into the water to be suspended turning your water cloudy if not green, until it settles, and once again you have a covering of Dead Algae Dust on the floor of your pool. But when the filter valve is in Waste mode, you will bypass the filter and the return line altogether and your vacuumed algae silt will be sucked up into the valve, and OUT the valve to waste. And just like that all of the algae dust is vacuumed out and did not Blow back into your pool.
I just had a situation where we cleaned up such a pool where the customer couldn't get the algae out, we came out and vacuumed to waste. And I by chance took a picture of the partially vacuumed pool and Dust remaining.

If you don't have a multipart valve, then if the pool has no debris but only dust, then and ONLY then, should you use Backwash mode. You NEVER want to vacuum a pool with debris in Backwash mode! NEVER!!! Because the larger debris you are vacuuming will get caught in the internals of your lateral drain assembly or your DE Grids. then it will be very difficult to un clog them, which in turn will reduce proper water filtration efficiency! But the algae dust will be able to pass right on thru. If you have neither of these types of filters because you have a Cartridge style filer, the elements may be able to trap the dust, if it is still to fine to filter, you obtain an additional pool pump, place it pool side, and in the pump out the pump will be two vacuum hoses and away you go.

Good Luck

sand filters only have limited filtration. If the dirt is very fine such as dead algae, it will go right through the filter. Use a clarifier to help give the filter finer filtration. Do not vacuum until you can see what you are doing, then vacuum on 'waste'

iuf youre just opening and pools Green ? then Yes you want to pull the algae dust Dwn to bottom.. So you can vacum to waste ..Sand cannot filter algae particles it will just come thru the returns you can view it ...

For regular vacuuming, these pool filters should be set to the "filter" setting filter control valve. However, if you have a heavy load or the debris seems to be coming back into the water, you can try vacuuming with the filter set to the "waste" position. This will bypass the filter and vacuum directly out to waste.

if the debris your are trying to vacuum up is more fine than the sand can catch (specifically with algae or dead algae), it will blow right through the filter. there is a setting on your filter called either drain or waste. when using this setting the water completely bypasses the sand and comes straight out the back wash outlet. this is a rather tedious task as you lose alot of water, but it is the only way to remove this type of particulate. make sure the pool is overfilled when you begin. good luck!

dead algae is made up of microscopic particles. Most sand filters have trouble filtering out particles that small. I would recommend using a flocculent that will force the particles to settle to the bottom, you can then set the multiport valve to the "waste" setting and vacuum as much of the debris as possible. You can use an additive like fiberclear to help the filter clear the water up quicker.

As in most cases today, pumps and filters are not properly matched. The filter cannot retain such fine algae particles because they are being driven through the sand bed by an over zealous pump. I suggest that you let the dead algae settle to the bottom. top up water and use the vacuum to waste position on m.port valve. Vacuum in such a way that you dont stir up the debris.

It could be that there is algae growing inside the tank of the filter. You may want to vacuum to waste to get the algae out of the pool and clean out the filter. If you have a Cartridge or DE filter, you will need to do an acid bath for the grids. Take the internal parts out of the filter and clean with bleach water, use bleach water on the tank also.

If you have a sand filter, you will need to empty out the sand, clean internal parts and tank with bleach water then fill with new sand.