7/21/23 First I want to say that all three Oase Biomaster filters ,I paid for with my own money. They were not given to me to write a review. I have been running filter now with modified in canister prefilter setup and this mod is doing exactly what it is supposed to do as far as acting as a prefilter . Was a little concerned at first that their would be a bypass with the removal of prefilter pipe but that has been dispelled with inspection . As far as ANY air purge it is NON-existent .To any one who has this filter I would strongly suggest doing this mod. All it will cost is the price of two Tetra pond prefilter sponges approximately $5.50. I am going to try to add a picture of 55 gallon this filter is running on . Notice Sword plants to the left and see the flow coming out which was not there before. And yes I am running Co2 with an inline Co2 Art diffuser on Oase. Something that questionable customer service people at Oase said would cause purging . The bubble count is about 5 per second ,no fooling around injection quantity.5/19/23 Yesterday cleaned Biomaster 600 . Now that I have totally identified the cause of purging air from this filter I decided to try another Mod. I took another Tetra Pond pre filter and stuffed it into the empty prefilter housing . Made sure to keep it at the top of the housing so it is essentially up against the primer housing to avoid bypass. Of course left out prefilter pipe . The coarseness of the internal Tetra sponge coupled with the elimination of the pipe has made this canister SOOOO far a great filter. No purging ,no flow problems and I'm back to an easy clean internal prefilter. Regardless what happens with this latest mod I still have No purging and an improved flow rate . Tetra Pond sponge is still on in tank pickup tube . Think I will change rating to 5 stars. Again this is on a heavily planted overstocked 55 gallon that I rarely do water changes on . No amonia,no nitrites and Nitrates are down around 15ppm to 20ppm even with addition of ferts.4/18/23 Just want to let anyone looking at this that purging is GONE . In canister prefilter and pipe removal made all the difference in the world . Also drop checker for CO2 is looking better (more towards a lighter green color) without Oase doing it's air injection.Another Update 4/11/23 I will start with the positive and try not to mention the Oase corporate line on their canisters problems . One very telling thing in this whole Oase journey for me is the fact that owning the Biomaster 250 along with the 350 and the 600 showed me that Oase should have stopped at the 250. I am running that filter with a very fine sponge on the intake in the tank . It is solidly clogged with muck and it DOES NOT purge air at all . The pre filter in the filter housing is never dirty due to what is on the in tank pick up. Just recently picked up 3/4 inch pvc pipe Reemed out one end to fit on primer head ,drilled the living crap out of pipe (almost to point of breaking ) . Installed coarsest pre filter sponges on (swiss cheese looking pipe) and installed in 600 canister . Not any noticeable difference in performance ,flow wise and definitely did not help purge issue . I do have on 600 tank intake tube a VERY coarse Tetra Pond pre filter . What happens like the 250 the oase 600 canister pre filter never gets dirty . But still has the Purge issue . SOOO what I have done was removed every thing from the canister pre filter(wasn't doing anything anyhow ) and it improved flow also I could be wrong but it helped so far with purging . If I am correct maybe and I am saying MAYBE two problems have been cured .Reason for statement on flow is because I have a very heavily planted tank and plants are leaning over from current that never did before . No scientific gallon per hour check just and unscientific observation. I will definitely stay with Tetra Pond Filter prefilte arrangement as long as I am using this filter.https://www.amazon.com/gp/product/B002RBEGC8/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1Update 1/17/23 Well after extensive conversations by phone ,Email and Text with Adam Raab who has the title of "Aquatics Category Manager (North America) / Regional Sales " The answer to the well known air purging issue with these way over priced filters is to send them back to where you purchased it from . Apparently it looks like they either don't know how to fix them or they don't care to. So here it is ,if you inject CO2 in your tank you definitely DO NOT want this filter ,or if you like spending money to get your co2 refilled a lot more frequently because your Oase combo canister filter /air stone carried the co2 you just injected up out of your tank then this is the filter for you. Yes folks Oase has answered the question of where to put the air stone in your tank, its all incorporated into the return from your wonderful Oase all in one filter and oxygen delivery device. I can't understand where the positive reviews are coming from . It cannot be from customer service because when you speak to anyone from the company all you get like their filter is a lot of air unlike the filter theirs is hot. I have been in the tropical fish hobby since I was in my early teens . I am now 73 and I think I know a little something about keeping fish and equipment that goes with it . Unfortunately In 2019 I made the mistake of buying this joke of a canister filter the Biomaster 350. From day one it constantly purged air,early on that is to be expected ,but this thing never stopped. I tried a few things as far as the way the media is in the filter and that never made a difference . With all the air purging into the tank water looks like Sprite. I even went with a courser inside canister pre filter thinking that may be issue . No it was not . A couple of years ago when I called Oase about problem their answer was to send me a new head unit , cured nothing. They did admit that they had issue back then and that was reason for exchange head. I have to clean pre filter every two weeks which helps slightly . I just got off phone with Oase and now they say they don't have problem with 350's and when I asked why they sent me exchange head unit a couple years ago and admitted to issue then ,now they have no answer . All Oase is admitting to is the problem is with the 600 which is the exact same filter as the 350 except a little bigger otherwise identical. Before you go and spend good money on this do a lot of investigating ,Oh did I mention the flow out of this so called filter also sucks . Save your money and stay way from Oase. Looks like I am going back to Eheim canister I bought back in 1977 ,never again Oase. Probably if I said my problem was with a 600 they would have admitted to 350 being the problem. I think what Oase is proficient in is the art of Flimflam .Update 8/4/22 : If you have this filter get yourself a jar of silicone grease and silicone all of the gasket areas on this filter. Not just a like film of silicon but I'm talking about slopping this stuff on . I tried a light film ,did not work . Then I bought silicone grease in jar and went crazy with it at every gasket and joining parts this filter has,knock wood problem with air for the most part is gone. Still have a little purging but that is to be expected especially as filter gets dirty .From Amazon : Food Grade Food Grade Pure Silicone Grease, 2.0 fl. oz. (59 ml) JarUpdate 10/5/2022I have updated my rating because of one thing I have recently found out and had an Oase rep admit to it . If you use a foam prefilter on your in tank intake or in the case of a planted tank where there is some dead plant material on intake you will cause more and heavier air purging from filter. What is happening is the pump is causing cavitation inside the filter because of restriction at the intake . I haven't tried a very coarse foam pre filter but I have noticed that slightest restriction at in tank intake will cause increase in purging of air. My tank is very heavily planted and i have noticed it more often .Essentially the filter works well and is well built and that is reason for change in rating . Unless Oase gets a new arrangement for intake I would not use it on a tank with shrimp and such.