the issue here is phosphate or PO4. Assuming that the "red algae" you speak of is cyano it will take some time to eradicate.
A while ago I used tap water in my reef. I did this for a year straight and had no ill effects from it. What I did not see was that my tap water was high in PO4. PO4 in the wild is in high demand but short supply. PO4 will be adsorbed by most things in your tank. Sand, Live Rock, algae etc. In my tank for example, a years worth of PO4 was in my Live Rock and Sand until it could hold no more. At that point no more PO4 could be absorbed by my rock and it just stayed in the water column where other things could grab it, like cyano algae.
My tank became overrun by cyano. The thing to know here is that you can NEVER fully get rid of cyano as it exists in the air we breathe. All you can do is remove it's food source thereby making your tank a place where cyano cannot live.
The problem that you will have is as I mentioned PO4 is in high demand. As your rocks leach it out it will be grabbed up quickly by many things. You want it to be grabbed by something that you can throw away, thereby removing it from your system. For example, suppose some algae grabs it. As that algae dies it will rerelease the PO4 back into your tank, no good. If your cheato grabs it and you then throw the cheato away, you've removed the PO4 from your tank. Using PO4 removers in a reactor such as phosban is a great way to remove PO4.
One thing to keep in mind is that this process will likely take months. Be mindful of what you add to your tank because you don't want to add PO4. Unrinsed frozen food is VERY high in PO4 as are mysis. Many additives such as phyto plankton and cyclopeeze are also high in PO4. By removing lots and adding none you will speed up the process.
Another noteworthy mention is the bacteria killers and antiobiotics like red slime remover etc. These attack the symptom and not the problem. Because cyano is in the air those products only offer temporary solutions.
Frequent water changes, ciphoning out the cyano along with the other steps is your best bet to solving your problem. You can also try vodka or sugar if you're brave but do a lot of research before you try unorthodox steps like that.
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