Had to look up your grass and got a pleasant surprise. We call it St Augustine, so there is a lot of information about it. The reason you have weeds is you are doing your watering wrong. I suspect you are mowing wrong, too. Here is the complete package on St Augustine.

Watering
Water deeply and infrequently. Deeply means 1 inch (2.5 cm) all at one time. Infrequently means once per month when temps are below 60 degrees F (15 C). With temps above 70F (21C), water every 3 weeks. With temps above 80F (27C) water once every 2 weeks. With temps higher than 90F (32), water once per week. You will have to use some cans to measure how long it takes to apply 1 inch of water. If you were in the US I would tell you to time how long it takes to fill cat food or tuna cans. I'm not sure what you have in Portugal in the way of easily available cans about that tall. Fava beans??? The idea for this watering regimen is to get a lot of water deep in the soil for the health of the soil biology. Secondarily it is to allow the surface of the soil to dry out completely. What you are doing with 2x daily watering is germinating every weed seed that lands in your soil. If you back off on the frequency and allow the soil (and seeds) to dry out, they will not germinate. Many people have found that simply changing the watering frequency solved most of their lawn problems. Fish pond water is perfect for watering. Since you are watering 2x per day now, you cannot stop that immediately and go to once per month. Your grass roots are much too shallow for that to work. You will have to wean the grass off the frequent watering. Start with watering once per day for a few days, then skip a day. Watch the grass for signs of drying out. If you see it wilt, water immediately but water for a longer time period than you did previously. By March you should be up to once per month watering with much deeper roots. If you have trouble with water runoff, or really, any other issues with what I'm telling you, then be sure to write back before something happens.

Mowing
St Augustine should be mowed at your mowers highest setting all the time. There is never any reason to mow it lower than the highest setting. When St Augustine gets up to 5 or 6 inches high (13 to 15 cm), it will shade out and crowd out almost all of your grassy weeds including bermuda. At that height it will become too dense for most weeds to survive even if the seeds do sprout. There will be some broad leaf weeds, like clover, oxalis, dichondra, spurge, horse herb (look all these up on Google Images) which do survive. Those will have to be pulled or sprayed.

The problem with spraying St Augustine grass is the grass is sensitive to most herbicides. Atrazine is the one that seems to work against the weeds and not harm the St Augustine; however, I would be surprised if you could buy that in Europe.

So I would not do anything in the way of chemical weed control until you get switched over to proper watering and your grass gets up to the full 15+ cm tall.

Fertilizer
Since you are using pond water, you are getting some ammonia nitrogen from the fish waste products. You might not need to fertilize. If you do you might look into sources of inexpensive animal feed. I use alfalfa pellets (rabbit food) to fertilize my lawn. Apply at the rate of 20 pounds per 1,000 square feet (10 kilos per 100 square meters). You can apply that as often as you can afford to without fear of harming anything. Alfalfa is often the main ingredient in commercially bagged organic fertilizers, and it works great at improving your soil's health and feeding the grass. Other animal feeds that work are corn meal, corn gluten meal (those are two different products), soybean meal, and even used coffee grounds makes good organic fertilizer. The application rate is always 20 pounds per 1,000 for grain type organics. If you double it or triple it by accident, you should not have any problems other than the smell of the stuff decaying.

St Augustine makes an excellent lawn. As long as you can keep it watered in the summer heat, it will look great most of the year.

Thank you very much for your time!

With your comment I found that I was doing everything wrong.

I watered too much and I'm going to reduce watering for one time a day, then one time a week and then one time a month.

I'm mowing wrong too. I always used the lower setting to cut all the weedings. From now on I will use the highest setting.

With fertilizing, I won't do anything for now.

Just making the changes to watering and mowing will dramatically improve your lawn. Grassy weeds should disappear quickly. Broad leaf weeds might take a little longer. I've been taking care of St Augustine for about 25 years. The first 15 I was doing it wrong. Once I took the right advice, everything got better. If you find that the grass is not very dense, you will need to fertilize it. I'm hoping the pond water will be all you need, though.